The median gestational ages at fetal loss were 20.6 (range: 15.7–23.9) and 21.5 weeks (range: 16.1–23.9 weeks) in the amniocentesis and no-procedure groups, respectively. Using these data, we evaluated the cumulative risk of fetal loss by the number of weeks from prenatal diagnosis using a Kaplan-Meier plot (Fig. 1). There was a significantly higher risk for fetal loss in the no-procedure group for the period between 15 and 24 weeks after prenatal diagnosis compared with the amniocentesis group (log rank P<.001). There did not appear to be any temporal clustering in the loss rate closer to the time of the procedure in the amniocentesis group. The fetal loss rates in the two groups remained stable over the study period, with ranges from 0.8–1.4% and 0.6–1.2% in the amniocentesis group compared with the no-procedure group, respectively (details available from authors).
To determine if the differences in mean gestational age at prenatal diagnosis between the groups may have influenced the findings, we conducted a nested case–control study from the entire cohort. Each case that had an amniocentesis was matched to three cases that had no procedure, matching for gestational age (±6 days). The cases and their matched controls were chosen consecutively from the database. Due to the smaller numbers of controls with no procedures seen between 15 weeks and 17.9 weeks, we used a 1:1 matching ratio at those gestational ages. Due to this limitation in the early gestational ages, we could find a match for only 11,196 of the 11,695 (96%) of the amniocentesis group. Of 29,886 total subjects matched, there were 312 fetal losses (less than 24 weeks), 106 of 11,196 (0.94%) in the amniocentesis group and 206 of 18,690 (1.1%) in the matched control group, P=.11. Figure 2 shows the survival curve comparing the likelihood of surviving up to 24 weeks in the cases compared with the matched controls. There was a statistically significant increase in the rate of fetal loss in the no-procedure group compared with the amniocentesis group (log rank test P=.001). A similar pattern was seen for delivery at less than 28 weeks (data available from authors).
We evaluated potential variables that may be associated with fetal loss after amniocentesis. Among the women having amniocentesis, 202 had two needle insertions and one had three. The significant factors associated with a fetal loss after amniocentesis were the presence of a fetal structural abnormality, a history of a previous miscarriage, and a dark-brownish discoloration of the amniotic fluid (Table 3). Other discolorations of the amniotic fluid, including blood staining, were not associated with increased loss (data available from authors).
Assuming an alpha of 0.5, the study had 88% power to detect a 50% increase in fetal loss from amniocentesis above the background risk of 0.84% in the group not having amniocentesis. Table 4 summarizes the findings of published studies comparing amniocentesis-related loss rates with control groups that received no intervention.1,2,5,8–13 The differences in definitions of fetal loss and some of the limitations of the studies are depicted.
This study found no significant difference in fetal loss rates at less than 24 weeks between pregnancies after amniocentesis when compared with those without any invasive procedure. Our findings of a loss rate of 1:769 (95% CI 1:500 to 1:1429) adds to the growing body of evidence supporting the safety of amniocentesis.1,2,13 Eddleman et al1 reported a procedure-related fetal loss rate after second-trimester amniocentesis of 0.06% or 1:1600. Caughey et al13 reported a loss rate of 0.46 in a retrospective cohort study of 30,893 women undergoing amniocentesis. The procedure-related loss rate in the last 5 years of the latter study was 0.27%, or 1 in 370. In another retrospective study limited to women with abnormal serum screen from California, the rate of miscarriage after amniocentesis was 0.46%, which was lower than the 0.53% seen in women who did not have amniocentesis.2 The differences between studies on fetal loss after amniocentesis that included a control group that had no procedure are listed in Table 4. The inclusion criteria and definitions of fetal loss are variable. All of the listed studies did not explicitly define fetal loss as we did in the present study.
Our study has the added advantage of including a large number of women who had no invasive procedure as a comparison group. The fetal loss rate in both the amniocentesis and control groups from our study is, however, lower than that reported from most of the previous studies addressing this subject.5,6,14 One reason for this may be the inclusion of only women seen from 15 weeks onwards. Because this inclusion criterion was applied equally to both groups, we believe our conclusions to be reasonable and valid. Using high-resolution ultrasound guidance and limiting our needle size for the procedure to 22 gauge may also have contributed to the lower loss rate in our study (Alfirevic Z, Tabor A. Pregnancy loss rates after midtrimester amniocentesis [letter]. Obstet Gynecol 2007;109:1203–4).
When we considered the risk of fetal loss from the time of prenatal diagnosis, we found a higher risk in the group that had no procedure compared with the amniocentesis group. This higher risk persisted throughout the period between 15 weeks and 24 weeks. Although this finding does not imply that the amniocentesis procedure is without risk, it suggests that the risk is not significantly different from the background risk for spontaneous miscarriage. From a public health standpoint, a 0.13% procedure-related loss rate may become clinically significant when applied to a large population, particularly if the procedure in question is (or is anticipated to be) performed with increasing frequency.
The only significant factors associated with fetal loss in the analysis limited to the amniocentesis group were a brownish discoloration of the amniotic fluid, the presence of a fetal abnormality, and a history of miscarriage. This finding of an association between brownish amniotic fluid color and fetal loss is similar to that reported in previous studies.5,15 We, however, believe the discoloration to be secondary to pre-existing intraamniotic bleeding and subsequent amniorrhexis from enzymatic digestion of membranes, rather than from the amniocentesis procedure per se. Unlike previous reports, we found no association in this study between the number of needle insertions, transplacental passage, maternal age, or pregnancy termination and fetal loss after amniocentesis.5,16 In the subgroup analysis, the only group with a significantly different fetal loss rate compared with the group that had no procedure was in the group of women undergoing amniocentesis with normal serum screen for aneuploidy. Despite adjusting for other potential confounders including maternal age and the presence of a fetal anomaly, the difference remained statistically significant. The mechanism for this finding is unclear, but the loss rate in this subgroup of 0.17%, or 1:588, is still lower than the 1:200 suggested by the CDC.
Our study is not without limitations. These include the retrospective design and the possibility of selective follow-up in the amniocentesis group. The latter should lead to a detection of a higher loss rate in the amniocentesis group; therefore, our findings may be a conservative estimate if this is truly the case. The follow-up schedule described has been in place in our center for many years and was not designed specifically for the current study. Therefore, a differential follow-up is an unlikely reason for our findings. Although a randomized control trial would be ideal in determining the fetal loss rate associated with amniocentesis, we agree with Eddleman et al1 that such a study will be impractical with the present prenatal diagnostic practice patterns. The only randomized trial addressing this subject was performed between 1982 and 1984 and enrolled only low-risk women under 35 years of age. Those investigators reported a spontaneous loss rate of 1.7% in the amniocentesis group compared with 1% in the control group.5 In addition, an 18-gauge needle was used for the amniocentesis in this study by Tabor et al. In our cohort, documentation of the exact timing of the fetal loss was present in over 90% of those who had a fetal loss. We agree with the opinion of Nicolaides that this information is important in reviewing the accuracy of studies addressing the loss rate from procedures (Nicolaides K. Pregnancy loss rates after midtrimester amniocentesis [letter]. Obstet Gynecol 2007;109:780).
A recent systematic review has suggested matching for gestational age between the control group and those undergoing amniocentesis as a solution to this potential for bias.17 However, matched case-control studies have the disadvantages of introducing potential selection bias into the study and of not providing reliable incidence rates for some of the outcomes being studied because they are already fixed by the matching process.18 Because the women in the amniocentesis group were enrolled at a significantly earlier gestational age than those in the control group, we believed that this could bias the results in favor of a higher loss rate in the amniocentesis group compared with the control group, particularly because previous studies have shown that the higher miscarriage rates occur in the earlier gestational ages than later.19 We explored this possibility by performing a nested case-control study by matching for gestational age at prenatal diagnosis. The results of this analysis revealed a higher loss rate in the group that had no procedure compared with the amniocentesis group. The matched analysis was limited by fewer numbers of controls in the earlier gestational ages between 15 and 17.9 weeks. This study is not the first to report a lower fetal loss in the amniocentesis group compared with those who had no procedure.2 Similarly, in the adjusted model from the report by Eddleman et al,1 the odds of pregnancy loss were lower in the women who had amniocentesis compared with those who did not. The suggested reason for this finding is that, given a high association between chromosomal abnormalities and fetal loss and since the majority of pregnancies with such abnormalities are terminated after amniocentesis, a higher loss rate is seen in the group that had no procedure due to a relatively higher proportion of pregnancies with aneuploidy in those who had no invasive procedure. The difficulty with equal comparison is further confounded by the lack of chromosomal analysis in a high proportion of spontaneous fetal losses. One possible mechanism for adjusting for this potential bias is to estimate the expected rates of different chromosomal abnormalities in the control group and the expected loss rates in such pregnancies—for example, 40% of Down syndrome pregnancies will spontaneously miscarry by 24 weeks19—and, finally, reduce the loss rate in the no-procedure group by this factor. Because we have no reliable data about the incidence of chromosomal abnormalities and their loss rates in women who had no procedure in our population, we cannot perform such adjustments.
In conclusion, our study found the fetal loss rate after amniocentesis to be lower than the 1 in 200 risk suggested by the CDC and ACOG,3,4 and in our cohort is estimated to be 1 in 769. We also found that the total fetal loss rate after amniocentesis is not significantly higher than the risk in women who had no invasive procedure. This information will be useful for counseling women requesting amniocentesis. Other centers might consider validating these findings before adopting the reported rate for their unit, particularly if the center or patient population appears unlike ours.
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14. Randomised trial to assess safety and fetal outcome of early and midtrimester amniocentesis. The Canadian Early and Mid-trimester Amniocentesis Trial (CEMAT) Group. Lancet 1998;351:242–7.
15. Golbus MS, Stephens JD, Cann HM, Mann J, Hensleigh PA. Rh isoimmunization following genetic amniocentesis. Prenat Diagn 1982;2:149–56.
16. Papantoniou NE, Daskalakis GJ, Tziotis JG, Kitmirides SJ, Mesogitis SA, Antsaklis AJ. Risk factors predisposing to fetal loss following a second trimester amniocentesis. BJOG 2001;108:1053–6.
17. Mujezinovic F, Alfirevic Z. Procedure-related complications of amniocentesis and chorionic villus sampling: a systematic review. Obstet Gynecol 2007;110:687–94.
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© 2008 The American College of Obstetricians and Gynecologists
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