Obstetrics & Gynecology:
Weight Gain in Women of Normal Weight Before Pregnancy: Complications in Pregnancy or Delivery and Birth Outcome
Thorsdottir, Inga PhD, RD, RN; Torfadottir, Johanna E. MSc; Birgisdottir, Bryndis E. MSc, RD; Geirsson, Reynir T. PhD, MD
Unit for Nutrition Research, Department of Food Science, and Department of Obstetrics and Gynecology, Landspitali‐University Hospital, University of Iceland, Reykjavik, Iceland.
Address reprint requests to: Inga Thorsdottir, PhD, Landspitali‐University Hospital, Unit for Nutrition Research, Reykjavik, IS‐101, Iceland; E‐mail: firstname.lastname@example.org.
Supported by the Fund for Research and Education, the Research Fund at the University of Iceland, and the Icelandic Ministry of Agriculture.
Received August 1, 2001. Received in revised form December 3, 2001. Accepted January 10, 2002.
OBJECTIVE: To investigate the relation between gestational weight gain in women of normal prepregnant weight and complications during pregnancy and delivery in a population with high gestational weight gain and birth weight.
METHODS: Healthy women (n = 615) of normal weight before pregnancy (body mass index 19.5–25.5 kg/m2) were randomly selected. Maternity records gave information on age, height, prepregnant weight, gestational weight gain, parity, smoking, gestational hypertension and diabetes, preeclampsia, delivery complications, and infants' birth size and health.
RESULTS: The mean weight gain in pregnancy was 16.8 ± 4.9 kg (mean ± standard deviation). A total of 26.4% of the women had complications, either in pregnancy (9.1%) or delivery (17.3%). Women gaining weight according to the recommendation of the Institute of Medicine (11.5–16.0 kg) had lower frequency of pregnancy‐delivery complications than women gaining more than 20.0 kg (P = .017), but did not differ significantly from those gaining 16–20 kg (P > .05). When dividing weight gain in pregnancy into quintiles, a relative risk of 2.69 (95% confidence interval 1.01, 7.18, P = .048) was found for complications in pregnancy in the fourth quintile (17.9–20.8 kg), using the second quintile as reference (12.5–15.5 kg). The mean birth weight was 3778 ± 496 g. A low weight gain in pregnancy (less than 11.5 kg) was associated with an increased frequency of infants weighing less than 3500 g at birth (P < .01).
CONCLUSION: A gestational weight gain of 11.5–16.0 kg (Institute of Medicine recommendation) for women of normal prepregnant weight is related to the lowest risk for pregnancy‐delivery complications. In the population studied, the upper limit might be higher (up to 18 kg), and low weight gain should be avoided to optimize birth outcome.
Optimal weight gain in pregnancy is a matter of debate because of its relation to optimal perinatal outcome, better health later in life, normal adult weight, and nonobesity, and better health in pregnancy and delivery with a minimum of complications.1–12 The recommendation for weight gain in pregnancy from the Institute of Medicine (IOM) for women of normal weight before pregnancy (ie, 11.5–16.0 kg2) has been commented on by different researchers and supposed to be appropriate,8 too low,6 or too high.5
Bracero and Byrne found that women of normal weight before pregnancy had an optimal perinatal outcome at a gestational weight gain higher than the IOM recommendation or between 14.2 and 18.4 kg.6 High birth weight has repeatedly been related to better health in infancy11 and adult life,12 and recently even to mothers' later health,13 but there is a positive correlation between gestational weight gain and birth weight.3,9,14–16 The optimal birth weight is considered to be in the range of 3500–4500 g.1 On the other hand, Feig and Naylor5 suggested that women of normal weight before pregnancy should not gain more than 11.4 kg, one reason being increased adult obesity in Western countries. However, it was recently reported that most women of normal weight before pregnancy regain their normal weight within 1½–2 years after delivery, irrespective of gestational weight gain.3 Prepregnant weight was most indicative of women's weight 1½–2 years postpartum. High weight gain in pregnancy has been connected to gestational complications (ie, hypertension, diabetes and preeclampsia), complications in delivery,2,7,9,10,17 and macrosomia.18 More knowledge is needed about gestational weight gain among women of normal weight before pregnancy and how this relates to complications in pregnancy and at delivery. The relation has to be studied in populations of high birth weight and high weight gain in pregnancy. Such knowledge will serve as a guide to optimal gestational weight gain for improving maternal and neonatal health. This will also provide an opportunity to evaluate the IOM recommendation. The aim of the present study was to investigate the relationship between gestational weight gain among women of normal weight before pregnancy and complications in pregnancy and delivery in a population of high gestational weight gain and high birth weight.
MATERIALS AND METHODS
The target group included women with normal weight before pregnancy (n = 615) who were randomly selected within 1 year (1998), according to the study participation criteria, for information on prepregnancy, pregnancy, and delivery factors as well as birth outcome. All were healthy before pregnancy and had no history of diabetes, hypertension, cardiovascular disease, or thyroid problems. They were 20–40 years old, had a prepregnant body mass index of 19.5–25.5 kg/m2 and delivered singleton infants after a 38‐ to 43‐week gestation, based on routine fetal biometry at an 18–20‐week ultrasound examination. All received early and regular antenatal care.
Information on the 614 women (99.5%) who were willing to participate after receiving written information about the study, and on their offspring, was collected, but one woman decided not to participate. The participation in the study was high, but participation in the study demanded no extra data collection as the information was collected and recorded by the prenatal care unit and maternity ward. The sample size, which constituted 14.7% of all births in the country during the study year, was calculated to give at least a power of 90 and P < .05, and to detect an effect of 1 kg weight gain in pregnancy. The study was approved by the Ethical Committee of the University Hospital and the Icelandic Data Protection Commission.
From maternity records at the Department of Obstetrics and Gynecology at Landspitali‐University Hospital, Iceland, information was collected on maternal age, height, marital status, smoking, parity, prepregnant weight, gestational weight gain, gestational hypertension, preeclampsia, and gestational diabetes. Maternal prepregnant weight was self‐reported, which is reliable for women of normal weight, and the risk of underestimation was therefore minimal.19–22 Gestational hypertension was defined by World Health Organization criteria, as a blood pressure of 140/90 mm Hg or more on two occasions 6 hours apart.23 Preeclampsia was diagnosed by the presence of proteinuria and gestational hypertension. In the analysis, women with preeclampsia were not included with the gestational hypertension group. Gestational diabetes was diagnosed by World Health Organization criteria, by a fasting blood glucose more than 6.1 mmol/L or 120‐min value more than 7.8 mmol/L after a 75‐g glucose load.24 This test was performed if urinanalysis data demonstrated glucosuria. Delivery complications (ie, forceps delivery, vacuum extraction, and cesarean delivery) were also recorded. Birth weight and length, head circumference, and the infant's condition at birth were recorded. The infant was defined as healthy if no diagnosis of congenital anomaly or disease, including asphyxia or convulsions, were recorded at birth. Complete data were found for all these factors, for every woman in the study, except for smoking habits, where data were missing for 28 women.
The data are described by values of mean and standard deviation as well as percentages. The significance between two means was calculated by t test or Mann‐Whitney‐Wilcoxon test for non‐normally distributed data. The χ2 test was used in comparison analyses, whereas correlations were evaluated by Pearson and Spearman tests. When analyzing women with hypertension, women with preeclampsia were also excluded from the comparison group. The women were divided into four weight gain groups (ie, less than 11.5 kg, 11.5–16.0 kg [IOM recommendation for women of normal weight before pregnancy], 16.1–20.0 kg, and more than 20.0 kg). Trend analysis for different complications across the four groups of weight gain in pregnancy was done using the χ2 trend test.25 Post hoc tests were performed using least significant difference analysis of variance. To minimize inequality in numbers within weight gain groups and to see more clearly if there was an increase in risk at a certain weight gain, quintiles were used as a basis for analysis of relative risk (ie, less than 12.5 kg, 12.5–15.5 kg, 15.6–17.8 kg, 17.9–20.8 kg, and more than 20.8 kg), using the second group as the reference group (12.5–15.5 kg) as it is most similar to the IOM recommendation. In logistic regression analysis, data were adjusted for age, height, parity, gestational length, and birth weight. The SPSS program 9 (SPSS Inc., Chicago, IL) was used for statistical analysis and P < .05 was considered statistically significant.
Table 1 gives maternal and neonatal characteristics. Of the women, 61% were parous, 9% were single, and 17% were smokers. There were 162 women (26.4%) who developed complications, of whom one‐third in pregnancy (n = 56) and two‐thirds at delivery (n = 106). Figure 1 shows the proportion of women with pregnancy‐delivery complications within 2‐kg intervals of weight gain. Table 2 shows characteristics and neonatal size among women without complications and in three groups: complications in pregnancy or delivery, complications in pregnancy only, and complications in delivery only, as well as level of statistical difference (P) between each complication group and those without. Preeclamptic women (n = 22) and nonpreeclamptic women (n = 592) did not differ in gestational weight gain measured in kg, but there was a difference in proportional weight gain between these groups (31 ± 7% versus 27 ± 8%, P = .021). Average weight gain in gestational hypertensive subjects (n = 32) was 18.8 ± 5.3 kg, compared with 16.7 ± 4.9 kg in nongestational hypertensive women (n = 560) (P < .016). A similar difference in proportional weight gain as in preeclampsia was found between women developing gestational hypertension and those who did not (P < .023). Only two women in the study developed gestational diabetes, and both had low weight gain during pregnancy (less than 11.5 kg).
Table 3 shows complications in pregnancy and delivery and the outcome for infants by maternal weight gain categories. There was a lower frequency in pregnancy‐delivery complications (P = .017), pregnancy complications only (P = .042), as well as higher frequency of normal delivery (P = .033) among women gaining weight as recommended by the IOM (11.5–16.0 kg) compared with those gaining more than 20.0 kg in pregnancy. There was no significant difference between the reference group and those gaining 16.1–20.0 kg (P > .05). A significant difference was found in gestational hypertension between those gaining less than 11.5 kg in pregnancy and those gaining more than 20.0 kg (P = .026) and forceps delivery and vacuum extraction rates between those gaining 16.1–20.0 kg and those gaining more than 20.0 kg (P = .033). There was a trend in higher frequency of pregnancy complications (P = .024) as well as in higher frequency of gestational hypertension (P = .026) with increasing weight gain.
Table 4 shows the relative risk for complications when weight gain in pregnancy was divided into quintiles. There was only a significant increase in relative risk for pregnancy‐delivery complications when gestational weight gain was above 20.8 kg compared with the reference group of 12.5–15.5 kg, and for complications in pregnancy only with gestational weight gain of 17.9–20.8 kg and more than 20.8 kg. This means an increased risk for complications in pregnancy above weight gain of 17.9 kg. However, the lowest risk was always found in the reference group or in the group gaining more than 15.5–17.8 kg.
All 614 subjects in the study delivered live births, and 97.6% of the infants were defined healthy. Maternal weight gain correlated with birth weight (r = 0.26, P < .001) as well as birth length (r = 0.23, P < .001). Table 2 shows size at birth of infants for women with complications in pregnancy or delivery and those without. Women with preeclampsia had infants weighing on average 311 g less than infants of mothers without preeclampsia (P = .004), but there was no difference for those whose mothers had gestational hypertension (P = .454). Women delivering macrosomic infants (more than 4500 g) had delivery complications in 27.9% of cases, compared with 16.5% for others (P = .056). Table 3 shows a higher frequency of infants weighing less than 3500 g at birth among those gaining less than 11.5 kg in pregnancy, compared with the other weight gain groups (P < .01). This difference was also significant between those gaining weight according to the recommendation (11.5–16.0 kg) and those gaining more than 20.0 kg (P = .01). However, there was a significant difference in frequency of macrosomic infants between those gaining weight according to the recommendation and those gaining 16.1–20.0 kg as well as with those gaining more than 20.0 kg (P < .05). By quintile groups, the relative risk of delivering an infant weighing more than 4500 g was not increased from the reference group until at a weight gain of 17.9–20.8 kg when it was 3.54 (P = .017, 95% confidence interval 1.26, 9.97).
Pregnancy or delivery complications were found among 37.2% of mothers having their first infant, compared with 19.4% of multiparous women (P < .001). The same figures for pregnancy complications only were 13.6% and 6.2% (P = .002) and 23.6% and 13.2% for delivery complications only (P = .001). Preeclampsia was diagnosed in 6.6% of women having their first infant and 1.6% of multiparous women (P = .002), whereas the difference was not significant between the same groups in gestational hypertension (7.1% versus 4.4%, P = .190). There was no difference in the frequency of smokers and single mothers between the groups of women with complications and the group with no complications (P > .05).
The frequency of pregnancy‐delivery complications for women of normal weight before pregnancy, in a population with high gestational weight gain and birth weight, was highest among women gaining more than 20.0 kg in pregnancy. On average, the women gained 16.8 kg, which is just above the upper limit of weight gain recommended by the IOM for women of normal weight before pregnancy (11.5–16.0 kg).26 The average weight gain, as a proportion of the women's weight before pregnancy, was 27%. These figures are the highest values describing average weight gain in pregnancy found in the literature for women of normal weight before pregnancy.7,11,16,17,27
Every fourth woman in the study had complications during pregnancy or delivery, and the frequency was similar in this study as seen in normal materials from other studies.8,16,17,28 As the population studied is tall,29 the lack of a higher frequency of complications despite higher proportional weight gain in pregnancy indicates that taller women, compared with women of lower stature, adapt well to a higher proportional weight gain in pregnancy. The exception in frequency of complications is the few cases of gestational diabetes, but the incidence and prevalence of type 1 and type 2 diabetes, respectively, is also lower in the Icelandic population than among genetically related nations.30,31 This has been attributed to environmental factors, such as different food habits and the composition of food in Iceland, as well as the high birth weight.3,32–34
Women who had pregnancy‐delivery complications were shorter, supporting that mother's height has a protectional effect against complications.35 The women having complications also gained slightly more weight proportionally, which has been found in earlier studies.7 In the current study, this was mainly due to higher weight gain among the women with pregnancy complications (ie, preeclampsia and hypertension). Women having these complications in pregnancy have been shown to be at higher risk of hypertension and heart disease later in life. It is noteworthy that the complications in delivery in the present study were independent of weight gain in pregnancy as well as proportional weight gain, but a relationship has been shown in other studies for cesarean delivery.2,7
The frequency of complications was lowest in the group with the recommended IOM weight gain (11.5–16.0 kg). Compared with the group gaining more than 20.0 kg, it was significantly lower for both total complications and complications in pregnancy, and the frequency of normal delivery was also higher. Moreover, no significant difference was found in complications, or frequency of normal delivery between those who gained weight according to the recommendation (11.5–16.0 kg), and those who gained 16.1–20.0 kg. This shows that the recommendation given by the IOM is valid with regard to a minimum of complications in pregnancy and delivery, and that weight gain even up to 20.0 kg is not associated with complications. To define where in the range of 16.1–20.0 kg weight gain in pregnancy the risk increases, the women were divided into quintiles according to weight gain in pregnancy. This showed a tripling of the risk of complications in pregnancy after a weight gain of approximately 18 kg (fourth and fifth quintile), compared with a range of weight gain close to the IOM recommendation, 12.5–15.5 kg (second quintile). These results indicate that in the population studied, an upper limit on weight gain of 18 kg during pregnancy among women of normal weight before pregnancy is not unhealthy with regard to complications in pregnancy or delivery. Bracero and Byrne found a similar upper limit (18.4 kg) for weight gain in pregnancy to be optimal for perinatal outcome.6
An important reason why weight gain in pregnancy should be monitored is the relation between weight gain in pregnancy and birth weight, which has been shown in studies within different countries. Interestingly, when comparing results from populations with similar ethnic backgrounds, such as in the United States (seven studies), Australia (one study), Canada (two studies), and Sweden (one study),11 as well as an earlier study of our group36 and the present study from Iceland, the clear increase in birth weight with increasing weight gain in pregnancy could still be seen (r = 0.66, P = .015) (Figure 2). This is important as studies from around the world have shown a relation between low size at birth of infants and an increased risk of infant mortality2,11 as well as a later risk of several metabolic diseases, such as type 2 diabetes and coronary heart disease.12 In this study, an increased trend toward a higher frequency of infants weighing more than 3500 g with increased weight gain in pregnancy was found, but 3500–4500 g has been classified as optimal birth weight range.1 The frequency of children born weighing less than 3500 g was significantly higher in the first (less than 12.5 kg) than in the second (12.5–15.5 kg) quintile of weight gain in pregnancy. This indicates that the recommendations should not be too low. There was also a higher frequency of macrosomic infants (more than 4500 g) with increased weight gain, but the risk was not significantly increased until after at least a weight gain of 18 kg, similar to the finding of increased pregnancy complications. As delivery complications seem not to be strongly associated with weight gain in pregnancy, they probably should not be a strong decisive factor when the recommendations are evaluated.
Taller women have earlier been shown to give birth to heavier infants,9,16,36 and they gain more weight in pregnancy. The birth size of infants in this study is at the upper end of the scale worldwide, as has been shown before.3,33 Women having pregnancy‐delivery complications had longer and thinner infants (low ponderal index), compared with other women, which has not been shown before. For women with pregnancy complications only, the birth weight and ponderal index were lower than for the group with no complications during pregnancy. This confirms that complications in pregnancy negatively affect the birth size of infants, which is known37 and is of concern regarding later metabolic disease.12 The women with complications in delivery only, had infants with a larger head circumference than women with no complications in delivery, as expected. A recent study showed that women having small infants may be at higher risk of dying from heart disease than women having larger infants.13 This seems to support the importance of measures aimed at increasing the birth weight to an optimal level, and one way might be the optimal weight gain of women in pregnancy.
The IOM recommendations are built on a range of studies on women of different prepregnant weight.26 The present study, which is the first to investigate complications in pregnancy and delivery as well as birth outcome in a normal population with high gestational weight gain and high birth weight, supports the IOM recommendation. In the population studied, a weight gain up to 18 kg did not link to a higher risk of complications. Other studies using different methods have also supported the recommendations.2,8,26,38 The recommendations on weight gain in pregnancy have, however, been criticized as being too liberal in regard to later obesity of the mother.5 An earlier study on Icelandic women of normal weight before pregnancy who gained either 9–15 kg or 18–24 kg in pregnancy showed that 89% of them had reached normal weight again 18–24 months postpartum, regardless of weight gain in pregnancy. However, those gaining more weight, weighed 1.5 kg more than at the beginning of their pregnancy, indicating that there may be increase in body weight with an increased number of children. However, the incidence of obesity was low in the group of women gaining up to 24 kg.3 Furthermore, other factors than weight gain in pregnancy seem to be of more importance for weight after pregnancy.39
Even though a weight gain of 18 kg may be appropriate regarding pregnancy complications and the attainment of adequate birth size,6 it should be kept in mind that the reference values for gestational weight gain for women of normal weight before pregnancy can be different in different populations. An unnecessary low upper limit should be avoided as it has no beneficial effect on health and may even have negative consequences for women's well‐being in the form of stress and worries if they exceed the given weight,40 and it may also restrict optimal birth weight.2 The results of the present study also show the negative effects of low weight gain in pregnancy (ie, fewer infants in the optimal birth weight range and a tendency towards more complications in pregnancy and delivery, although the latter was not significant).
It is important to have recommendations on weight gain in pregnancy as a guideline for pregnant women with the health of both mother and infant in mind. However, women should not be pressured to fulfill the recommendations, but giving no guidance on gestational weight gain, as has been suggested,41 is not right. The recommendations used must be built on scientific knowledge about the association between weight gain in pregnancy and the health of the mother and the child, both during pregnancy and later in life. The importance of monitoring women's weight gain throughout their pregnancy should not be underestimated.
This study shows that a gestational weight gain of 11.5–16.0 kg for women of normal prepregnant weight is a valid reference regarding complications during pregnancy and delivery. In the population studied, the upper limit might even be higher (up to18 kg), and low weight gain should be avoided to optimize birth outcome.
1. Lederman SA. Pregnancy weight gain and postpartum loss: Avoiding obesity while optimizing the growth and development of the fetus. J Am Med Womens Assoc 2001;56:53–8.
2. Abrams B, Altman SL, Pickett KE. Pregnancy weight gain: Still controversial. Am J Clin Nutr 2000;71:1233S–41S.
3. Thorsdottir I, Birgisdottir BE. Different weight gain in women of normal weight before pregnancy: Postpartum weight and birth weight. Obstet Gynecol 1998;92:377–83.
4. Agarwal DK, Agarwal KN, Satya K, Agarwal S. Weight gain during pregnancy—A key factor in perinatal and infant mortality. Indian Pediatr 1998;35:733–43.
5. Feig DS, Naylor CD. Eating for two: Are guidelines for weight gain during pregnancy too liberal? Lancet 1998; 351:1054–5.
6. Bracero LA, Byrne DW. Optimal maternal weight gain during singleton pregnancy. Gynecol Obstet Invest 1998; 46:9–16.
7. Shepard MJ, Saftlas AF, Leo-Summers L, Bracken MB. Maternal anthropometric factors and risk of primary cesarean delivery. Am J Public Health 1998;88:1534–8.
8. Edwards LE, Hellerstedt WL, Alton IR, Story M, Himes JH. Pregnancy complications and birth outcomes in obese and normal-weight women: Effects of gestational weight change. Obstet Gynecol 1996;87:389–94.
9. Abrams B, Carmichael S, Selvin S. Factors associated with the pattern of maternal weight gain during pregnancy. Obstet Gynecol 1995;86:170–6.
10. Johnson JW, Longmate JA, Frentzen B. Excessive maternal weight and pregnancy outcome. Am J Obstet Gynecol 1992;167:353–70; discussion 370–2.
11. Johnston EM. Weight changes during pregnancy and the postpartum period. Prog Food Nutr Sci 1991;15:117–57.
12. Barker DJP. Mothers, babies and health in later life. Edinburgh: Churchill Livingstone; 1998.
13. Smith GD, Harding S, Rosato M. Relation between infants' birth weight and mothers' mortality: Prospective observational study. Br Med J 2000;320:839–40.
14. Shapiro C, Sutija VG, Bush J. Effect of maternal weight gain on infant birth weight. J Perinat Med 2000;28:428–31.
15. Hickey CA. Sociocultural and behavioral influences on weight gain during pregnancy. Am J Clin Nutr 2000;71:1364S–70S.
16. Mitchell MC, Lerner E. Weight gain and pregnancy outcome in underweight and normal weight women. J Am Diet Assoc 1989;89:634–8, 641.
17. Shepard MJ, Hellenbrand KG, Bracken MB. Proportional weight gain and complications of pregnancy, labor, and delivery in healthy women of normal prepregnant stature. Am J Obstet Gynecol 1986;155:947–54.
18. Hellerstedt WL, Himes JH, Story M, Alton IR, Edwards LE. The effects of cigarette smoking and gestational weight change on birth outcomes in obese and normal-weight women. Am J Public Health 1997;87:591–6.
19. Kuskowska-Wolk A, Karlsson P, Stolt M, Rossner S. The predictive validity of body mass index based on self-reported weight and height. Int J Obes 1989;13:441–53.
20. Gunderson EP, Abrams B. Epidemiology of gestational weight gain and body weight changes after pregnancy. Epidemiol Rev 2000;22:261–74.
21. Stewart A. The reliability and validity of self-reported weight and height. J Chronic Dis 1982;35:295–309.
22. Stunkard AJ. The accuracy of self-reported weights. Am J Clin Nutr 1981;34:1593–9.
23. Thadhani R, Stampfer MJ, Hunter DJ, Manson JE, Solomon CG, Curhan GC. High body mass index and hypercholesterolemia: Risk of hypertensive disorders of pregnancy. Obstet Gynecol 1999;94:543–50.
24. Alberti KG, Zimmet PZ. Definition, diagnosis and classification of diabetes mellitus and its complications. Part 1: Diagnosis and classification of diabetes mellitus provisional report of a WHO consultation. Diabet Med 1998; 15:539–53.
25. Bland M. An introduction to medical statistics. Oxford: Oxford University Press; 2001.
26. Keppel KG, Taffel SM. Pregnancy-related weight gain and retention: Implications of the 1990 Institute of Medicine guidelines. Am J Public Health 1993;83:1100–3.
27. Shepard MJ, Bakketeig LS, Jacobsen G, O'Connor T, Bracken MB. Maternal body mass, proportional weight gain, and fetal growth in parous women. Paediatr Perinat Epidemiol 1996;10:207–19.
28. Thadhani R, Ecker JL, Kettyle E, Sandler L, Frigoletto FD. Pulse pressure and risk of preeclampsia: A prospective study. Obstet Gynecol 2001;97:515–20.
29. World Health Organization. Monica study in Iceland. Age standardized figures for 1990–1992, 1993. Reykjavik, Iceland. Directorate of Health and the National Association for the Prevention of Heart Diseases.
30. Vilbergsson S, Sigurdsson G, Sigvaldason H, Hreidarsson AB, Sigfusson N. Prevalence and incidence of NIDDM in Iceland: Evidence for stable incidence among males and females 1967–1991—the Reykjavik Study. Diabet Med 1997;14:491–8.
31. Helgason T, Danielsen R, Thorsson AV. Incidence and prevalence of type 1 (insulin-dependent) diabetes mellitus in Icelandic children 1970–1989. Diabetologia 1992;35:880–3.
32. Steingrimdottir L. Nutrition in Iceland. Scan J Nutr 1993; 37:10–2.
33. Olsen SF, Joensen HD. High liveborn birth weights in the Faroes: A comparison between birth weights in the Faroes and in Denmark. J Epidemiol Community Health 1985; 39:27–32.
34. Thorsdottir I, Birgisdottir BE, Johannsdottir IM, Harris DP, Hill J, Steingrimsdottir L, et al. Different beta-casein fractions in Icelandic versus Scandinavian cow's milk may influence diabetogenicity of cow's milk in infancy and explain low incidence of insulin-dependent diabetes mellitus in Iceland. Pediatrics 2000;106:719–24.
35. McGuinness BJ, Trivedi AN. Maternal height as a risk factor for Caesarean section due to failure to progress in labour. Aust N Z J Obstet Gynaecol 1999;39:152–4.
36. Gunnlaugsson S, Geirsson RT. Weight gain among Icelandic women in pregnancy. Icelandic Med J 1992;78:115–7.
37. Beischer NA, Mackay EV, Colditz P. Obstetrics and the newborn. London: Sauber, 1997.
38. Parker JD, Abrams B. Prenatal weight gain advice: An examination of the recent prenatal weight gain recommendations of the Institute of Medicine. Obstet Gynecol 1992; 79:664–9.
39. Gunderson EP, Abrams B, Selvin S. The relative importance of gestational gain and maternal characteristics associated with the risk of becoming overweight after pregnancy. Int J Obes Relat Metab Disord 2000;24:1660–8.
40. Brownell KD, Fairburn CG. Eating disorders and obesity. A comprehensive handbook. New York: The Guildford Press, 1995.
41. Johnson JW, Yancey MK. A critique of the new recommendations for weight gain in pregnancy. Am J Obstet Gynecol 1996;174:254–8.
© 2002 The American College of Obstetricians and Gynecologists