Over the past 20 years, several hematologic disorders related to the risk of venous thromboembolism have been identified. These conditions are referred to as thrombophilia. Inherited thrombophilias are heterogeneous in terms of both the frequency in the normal population and the strength of their association with venous thromboembolism. Deficiencies of natural anticoagulants (ie, antithrombin, protein C, and protein S) and homozygosity for factor V Leiden mutation, prothrombin G20210A polymorphism, or double heterozygosity usually are considered rare and strong risk factors, whereas heterozygosity for factor V Leiden or prothrombin G20210A polymorphism are common and moderate thrombogenic states.1
Inherited thrombophilia is the leading cause of maternal venous thromboembolism.2 Although there is some evidence of the association of inherited maternal thrombophilia with pregnancy loss, to date inherited thrombophilia has not been proven to cause these complications.3 The magnitude of this association varies depending on the definition of fetal loss and the type of thrombophilia.2 As for venous thromboembolism, the risk of pregnancy loss may be higher in women who are homozygous than in those who are heterozygous for factor V Leiden. Four meta-analyses assessed the association between fetal loss and maternal factor V Leiden.4–7 However, even in these meta-analyses, no conclusion could be drawn as to the effect of the homozygous factor V Leiden genotype on early and late fetal loss owing to the low frequency of the defect (0.06–0.25%). Dudding included 10 homozygous cases in the first-trimester fetal-loss analysis and one in the analysis of late events and states that exclusion of these cases did not alter the results obtained.4 On the other hand, Rey and Kovalevsky did not specify the number of women who were factor V Leiden homozygous in their meta-analyses.5,6 In the meta-analysis carried out by Robertson, because of the limited data available, the analysis of factor V Leiden incorporated both homozygous and heterozygous carriers.7
We have, therefore, assessed the rate of early and late fetal loss in a cohort of French women carrying the homozygous factor V Leiden genotype and compared these frequencies with those observed in women with the heterozygous factor V Leiden mutation or with the normal genotype.
MATERIALS AND METHODS
Between December 1995 and February 1, 2007, consecutive, unrelated white women with the factor V Leiden homozygous mutation who had been pregnant at least once, irrespective of their clinical history. Patients with an objectively proven venous thromboembolism or individuals with a family history of venous thromboembolism were recruited from 10 regional hemostasis units. For reasons of comparison, we also included a group of women who were heterozygous for the factor V Leiden mutation and a group of noncarriers from thrombophilic families; both groups were enrolled in the Marseille Thrombophilia Centre (CEHT, LA Timone University Hospital, Marseille, France) during the same period. There were 1) unselected probands with objectively documented venous thromboembolism in whom a diagnosis of factor V Leiden mutation had been established or 2) consenting family members of these probands (with or without the factor V Leiden mutation).
Comprehensive clinical data forms were completed for each participant, with particular attention given to pregnancy, other risk factors for venous thromboembolism, antithrombotic treatments, and family history. Data were collected by personal interview and, when complementary information was needed, by questionnaire. Standard data collection forms were used at all participating centers. For the present analysis, women using thromboprophylaxis with aspirin or low-molecular-weight heparin or both during pregnancy were excluded. Pregnancy was defined by positive serum βhCG or ultrasound documentation or both. Early pregnancy loss was defined as pregnancy loss occurring within 12 weeks of gestation based on the last menstrual period. Late pregnancy loss was defined as loss occurring after 12 weeks. Severe preeclampsia was defined as blood pressure higher than 160/110 mm Hg, urinary protein excretion greater than 5 g/24 h, and a platelet count of less than 100,000/mm3. Abruptio placentae was defined by clinical criteria (vaginal or concealed bleeding, uterine tenderness, maternal shock or coagulopathy, fetal distress). Intrauterine fetal growth restriction was defined as a birth weight below the 5th percentile for gestational age.
All participants were screened for inherited or acquired thrombophilic abnormalities: antithrombin, protein C, protein S, lupus anticoagulant, immunoglobulin G and immunoglobulin M anticardiolipin antibodies, and prothrombin G20210A polymorphism.8 The participants’ blood groups were recorded. Written informed consent was obtained from each participant. The study was approved by the local institutional review boards (Montpellier and Marseille).
We analyzed maternal factor V Leiden results in association with two separate outcomes: 1) early fetal loss and 2) late fetal loss. Continuous variables were expressed as median and interquartile range. Categorical variables were expressed as frequencies and percentages. The Kruskall-Wallis test was used to compare continuous variables. Categorical variables were compared using the χ2 test or Fisher exact test when frequencies were below five. Results were given as odds ratios (OR) and 95% confidence intervals (CIs). P<.05 was considered significant. We also investigated the effect of a history of venous thromboembolism, parity, age (35 years or older), concomitant thrombophilia, and blood group on the risk of fetal loss. Statistical analyses were performed using SAS software 8.01 (SAS Institute, Inc., Cary, NC).
Of 139 women eligible for the present study, 44 were excluded (never been pregnant, n=25; thromboprophylaxis using aspirin or low-molecular-weight heparin during pregnancy, n=19). Finally, 95 women who were homozygous, 195 who were heterozygous, and 73 noncarriers who had been pregnant at least once and had never used thromboprophylaxis during pregnancy were analyzed (Table 1). The median age at inclusion was similar in the three groups. The number of pregnancies per woman ranged from one to five and was similar in the three groups (P=.80).
The percentage of women with early fetal loss was similar in the three groups (23.1%, 20.0%, and 21.9% for women who were homozygous, heterozygous, and noncarriers, respectively, P=.81) (Table 1). Only four women who were factor V Leiden homozygous, 12 who were heterozygous, and seven who were noncarriers had experienced two or more early fetal losses. The number of early fetal losses for the total pregnancies was also similar in the three groups (P=.96).
The risk of late fetal loss (Table 1) was higher in women who were factor V Leiden homozygous (13/95, 13.7%) compared with those who were noncarriers (1/73, 1.4%) (OR 11.41, 95% CI 1.46–89.46, P=.002). The risk was also higher in comparison with women who were heterozygous (6/195, 3.1%) (OR 4.99, 95% CI 1.83–13.6; P<.002). Conversely, the frequency of late fetal loss in women who were factor V Leiden heterozygous was not significantly different from that found in those who were noncarriers (6/195, 3.1%, compared with 1/73, 1.4%, P=.68). The percentage of pregnancies ending in late fetal loss was significantly higher in women who were factor V Leiden homozygous than in noncarriers (14/240 compared with 1/182, P=.003) or those who were heterozygous (P=.003). The frequency of adverse events in women who were heterozygous was not significantly different from that found in noncarriers (6/425, 1.4%, compared with 1/182, 0.5%, P=.68).
Among the 14 late fetal losses reported in the 95 women who were homozygous and the six late fetal losses in the 195 women who were heterozygous, seven (50%) and four (67%), respectively, occurred during the first pregnancy. The only late fetal loss in the noncarrier group occurred after a first successful pregnancy. Only one women who was factor V Leiden homozygous experienced two late fetal losses; no recurrence was observed in the other two groups. The median maternal age at the time of late fetal loss was similar in the three groups (P=.82, Table 1). None of the women had any adverse event after the age of 35 years. History of venous thromboembolism (P=.80) and parity (P=.15) did not affect the risk of fetal loss.
The frequency of women with preeclampsia, abruptio placentae, and intrauterine fetal growth restriction did not differ among the three groups (1/95 women who were homozygous compared with 3/195 who were heterozygous compared with 2/73 who were noncarriers, P=.69). None of the women in any group was deficient in antithrombin, protein C, or protein S, was a carrier of the prothrombin G20210A polymorphism, or had antiphospholipid antibodies.
In the three groups, the frequency of non-O blood group was statistically not different between women with late fetal loss and those without (women who were homozygous, 10% non-O compared with 21%, P=.42; heterozygous, 3% compared with 4%, P=.67; noncarriers, 0% compared with 5%, P=.34).
Among the 21 women who had experienced late fetal loss, taking into account all the previous and subsequent pregnancies, 10 women who were homozygous (23 pregnancies, range 1–5) and five who were heterozygous (13 pregnancies, range 1–2) had at least one successful pregnancy without thromboprophylaxis. The woman in the noncarrier group with a late fetal loss also had had two successful pregnancies. The live-birth rate was 80%, 84%, and 85%, respectively, for women who were homozygous, heterozygous, and noncarriers (P=.88). A history of pregnancy-associated venous thromboembolism was more frequent in women who were factor V Leiden homozygous compared with those who were heterozygous or noncarriers (P<.001) (Table 1). One venous thromboembolism occurred during a pregnancy, ending in fetal loss.
Given the relative rarity of the homozygous factor V Leiden mutation, a specific study including a sufficient number of participants must be used to analyze the effect of this type of mutation on the risk of fetal loss. Therefore, we conducted a retrospective, multicenter, cohort study including 95 women with childbearing potential carrying the homozygous factor V Leiden mutation, 195 women who were heterozygous, and 73 women who were noncarriers.
In Western countries, up to 18% of pregnancies end in fetal loss. In the large majority of cases, this is early fetal loss; in 0.5–0.8%, this is late fetal loss.9 In this study, we found an increased risk of late fetal loss in women who were factor V Leiden homozygous, whereas the prevalence of early fetal loss was not different from that usually found in the general population. This finding is not surprising—the earlier the pregnancy loss, the greater the likelihood of chromosome aberrations. Among first-trimester fetal losses, 50–90% show chromosomal abnormalities,10 whereas late fetal loss is a much rarer event and the risk factors are more easily identifiable. Recurrent early fetal losses are also likely to be more specific for underlying thrombophilia.
We failed to find any association between early fetal loss and the factor V Leiden mutation in either heterozygous or homozygous women. In a meta-analysis, Rey et al concluded that heterozygous factor V Leiden was associated with early fetal loss (OR 2.01, 95% CI 1.13–3.58, P=.02) but only for recurrent events.6 In another meta-analysis, conducted by Kovalevsky et al, the combined OR between first-trimester recurrent (two or more) pregnancy loss and factor V Leiden was 1.6 (95% CI 1.2–2.2, P=.002).5 Robertson found an OR of 1.91 (95% CI 1.01–3.6, P=.05) in women with factor V Leiden with recurrent pregnancy losses in the first trimester.7 For Dudding et al, the pooled OR for first-trimester fetal loss was heterogeneous. Even though some studies suggest an association between heterozygous factor V Leiden and early fetal loss, its magnitude is modest and concerns only recurrent events. Therefore, our study probably was not powerful enough to detect a very low risk mediated by factor V Leiden mutation because the percentage of recurrent early or late fetal loss was rather small.
Despite the increased risk of late fetal loss, it should be emphasized that the overall likelihood of a positive outcome is high in our series of women who were homozygous, with a live birth rate of 80%. The natural course of pregnancy among carriers of factor V Leiden has been investigated recently. After late fetal loss, the live-birth rate was 68% and 80% for carriers and noncarriers of factor V Leiden (or prothrombin G20210A polymorphism), respectively.11 According to Lindqvist, nulliparous women with one prior fetal loss and carrying the factor V Leiden mutation seem to have a good prognosis, with a 95–98% birth rate without treatment.12 Moreover, the prevalence of homozygous factor V Leiden status is consistent with Hardy-Weinberg equilibrium, arguing against significant selective loss of people who are factor V Leiden homozygous from the human population.13 Based on data from the Copenhagen City Heart Study, Juul reports that women who are factor V Leiden heterozygous and homozygous have, on average, 1.52 and 1.65 children, respectively, which does not differ from the average of 1.55 children in noncarriers.14 Van Dunne reports similar conclusions.15
The pathogenesis of thrombophilia-associated fetal loss, including factor V Leiden homozygosity, remains to be established.3,16 As early as 1997, Dizon-Townson et al found the fetal-carrier frequency of the heterozygous factor V Leiden mutation in miscarried fetuses to be twice that of the general population.17 More recently, it has been demonstrated that fetal gene defects increased the risk of pregnancy failure in factor V Leiden mice.18 Therefore, the fact that fetuses from women who were factor V Leiden homozygous are at least heterozygous for the factor V Leiden mutation could be important in pregnancy outcomes.19 As described for the factor V Leiden thrombotic phenotype, all these data suggest a major role of genetic modifiers or other thrombophilic defects on the risk of late fetal loss in women carrying the homozygous factor V Leiden mutation.
Our study has several limitations. Firstly, it was retrospective. Although other causes of fetal loss were not systematically sought, it is reasonable to assume that these are equally distributed among the groups. It is also unlikely that late fetal loss, which is a severe adverse event in comparison with early fetal loss, could have been omitted. Secondly, an important issue is participant selection. Women who were homozygous were included because they had a family or personal history of venous thromboembolism. For comparison, we chose to include two populations: a control group of women who were heterozygous and one of noncarriers. They were not from the general population but belonged to the thrombophilic families. If we assume that the risk of fetal loss, like the risk of venous thromboembolism, is higher in selected individuals from families with clear familial thrombophilia than in unselected individuals, it is important to select women who are heterozygous and noncarriers from these families and not from the general population. Individuals from thrombophilic families may have unknown thrombophilic defects that could affect the risk of fetal loss. In a previous study using a similar design, Tormene reports a frequency of late fetal loss in noncarriers higher than what was found in our study or in the general noncarrier population.20 However, the frequency of late fetal loss in our population of noncarriers is similar to that found in the general population.9 In any case, we believe that this selection of controls would have underestimated rather than overestimated the risk in our group of women who were factor V Leiden homozygous.
In conclusion, women who are homozygous for the factor V Leiden mutation have an 11-fold greater risk of late fetal loss in comparison with noncarriers. However, the overall likelihood of a positive outcome is high in our series. Whether or not thromboprophylaxis for women with thrombophilia and pregnancy complications is warranted is a matter of controversy.21,22 A recent study suggests that other marked thrombophilic defects, such as hereditary deficiencies in antithrombin, protein C, and protein S and combinations of these deficiencies with other mild thrombophilic disorders, are associated with a high risk of fetal loss that seems to be reduced by anticoagulant therapy during pregnancy.23 However, published uncontrolled studies should be viewed as hypothesis-generating and not as providing evidence to support systematic therapeutic intervention.3,24
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