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Relationship of twin zygosity and risk of preeclampsia.

OBJECTIVE
Twin gestations are known to be at higher risk for preeclampsia. One theory suggests that maternal recognition of fetal and trophoblastic tissues as foreign may be a factor. If that hypothesis is true, mothers carrying monozygous (MZ) gestations (ie a single fetal graft) might be predicted to have a lower rate of preeclampsia than those carrying dizygous (DZ) gestations. To evaluate this hypothesis, we compared the rate of preeclampsia in mothers with MZ and DZ twin gestations.


STUDY DESIGN
Seven hundred sixty-eight twin deliveries from 1994 to 1999 were reviewed. Placental pathology reports were reviewed to determine the chorionic state of each placenta. Monochorionic placentas were assumed to be MZ. Dichorionic placentas were categorized as DZ if the neonates were of different sexes or different blood types. Maternal and fetal data were abstracted from the medical records. Preeclampsia was defined by standard criteria of the National Institutes of Health Working Group on High Blood Pressure. Our analysis was limited to women with pregnancies reaching at least 30 weeks of gestation where zygosity could be determined.


RESULTS
Our analysis included 464 twin pregnancies, 154 MZ and 310 DZ. Among nulliparous women, the rate of preeclampsia was 15% (25/170) for DZ twins versus 20% (15/75) for MZ twins (P =.3). Among multiparous women, the rate was 8% (11/140) for DZ twins and 5% (4/79) for MZ twins (P =.4). In a logistic regression performed to control for confounding by maternal age, gestational age at delivery, assisted reproduction, and male sex, dizygotic state was associated with an odds ratio of 1.4 (95% CI = 0.5-3.9) for developing preeclampsia in nulliparous women and 1.2 in multiparous women (95% CI = 0.3-5.0).


CONCLUSIONS
: These results do not support the hypothesis that zygosity affects the rate of preeclampsia in twin gestations, though the number of subjects in our study was too small to allow definitive conclusions. Larger studies are needed to evaluate this finding.