Double trouble? Impact of frozen embryo transfer on the monozygotic twinning rate: a retrospective cohort study from 845
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ASSISTED REPRODUCTION TECHNOLOGIES
Double trouble? Impact of frozen embryo transfer on the monozygotic twinning rate: a retrospective cohort study from 8459 cycles Xitong Liu 1 & Ping Li 1 & Juanzi Shi 1 Received: 5 August 2020 / Accepted: 15 October 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Purpose To compare monozygotic twinning (MZT) rates in patients undergoing fresh embryo transfer (ET) and frozen embryo transfer. Methods All clinical pregnancies after single ET carried out in our IVF center between 2014 and 2018 (n = 8459) were retrospectively analyzed for the incidence of MZT. MZT rate was compared in women who underwent fresh ET (n = 3876) and frozen ET (n = 4583). Results There was a total of 120 MZT identified in the fresh ET group (3.10%) and 103 MZT in the frozen ET group (2.25%), which was significant (p = 0.015). In the univariate analysis, the risk of MZT was decreased with frozen embryo transfer (OR 0.72; 95% CI, 0.55–0.94, p = 0.016) and increased with mild stimulation protocol in the fresh cycle (OR 1.90; 95% CI, 1.04– 3.45, p = 0.036). Multivariable logistic regression revealed that frozen embryo transfer was associated with a significant decrease risk of MZT (adjusted OR 0.66; 95% CI, 0.46–0.90, p = 0.011). Conclusions Frozen ET is associated with a lower risk of MZT. Keywords Fresh embryo transfer . Frozen embryo transfer . Monozygotic twinning . In vitro fertilization
Introduction Monozygotic twinning (MZT) occurs when a single zygote divides early in embryonic development. MZT is classified according to their chorionicity and amniocity. If the embryo splitting occurs between day 0 and day 4 after fertilization, the resulting twins are dichorionic-diamniotic. If it occurs bet w e e n d a y 4 a n d d a y 8 , th e r e s u l t i n g t w i n s ar e monochorionic-diamniotic. If it occurs after day 8, the twins will be monochorionic-monoamniotic [1]. MZT occurs rarely in spontaneous conception, with an estimated incidence of 0.4% [2, 3]. Interestingly, the use of assisted reproductive techniques (ART) has been suggested to be associated with an increased risk of MZT, which is approximately 0.7–13.2% [4–6]. MZT carries a significantly higher risk for both fetuses and mothers compared with singleton and dichorionic twin * Juanzi Shi [email protected] 1
The Assisted Reproduction Center, The Assisted Reproduction Center, Northwest Women’s and Children’s Hospital, Xi’an, China
pregnancies, such as fetal growth restriction, preterm delivery, birth weight discordance, and perinatal mortality [7, 8]. The true incidence of MZT after IVF and in natural conception is likely underestimated, as dichorionic, diamniotic twins occurring after the double-embryo transfer is often assumed to be dizygous twins. The mechanisms producing MZT have remained unclear. Several factors may contribute to the increased rate of MZT observed after IVF, including ovarian hyperstimulation, assisted hatching (AH), maternal age, extended culture, and genetic factors. Theoretically, the
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