Controlled Ovarian Hyper-stimulation during IVF treatment does not increase the risk of preterm delivery compared to the transfer of frozen–thawed embryos in a natural cycle

Background Preterm delivery rates are significantly higher for IVF-conceived pregnancies compared with naturally conceived pregnancies, even following adjustment for relevant confounding factors. Furthermore, preterm delivery is reportedly more common in pregnancies from fresh embryo, rather than frozen-thawed embryo transfer (FET), suggesting that the controlled ovarian hyperstimulation (COH) environment may initiate preterm labour. Aims As prior studies have confirmed a positive correlation between the numbers of corpora lutea generated during COH and serum relaxin levels, a reported trigger for cervical remodelling and preterm labour, the aim of this study was to examine the hypothesis that preterm delivery rates will therefore be lower in mono-follicular FET cycles compared to multi-follicular COH/IVF cycles (primary hypothesis). Our secondary hypothesis was that COH/IVF cycles yielding higher numbers of oocytes will be associated with higher rates of preterm birth and subsequent lower birth weight than those producing lower numbers of oocytes. Methods Retrospective review of 702 singleton pregnancies resulting from IVF treatment (COH/IVF and FET) during the years 2008-2009 at a single private IVF centre in Adelaide, South Australia. Results There was no significant difference in rates of preterm labour between women undergoing COH with fresh embryo transfers compared with frozen embryo transfers (FETs), (11.59 vs 10%, P = 0.6011), nor was there any difference in the median gestation at delivery (39.4 vs 39.1 weeks, respectively, P = 0.1538). The rate of preterm delivery in both the fresh and frozen embryo transfer cohorts was higher than that observed in the general obstetric population (6.76%). A weak but marginally statistically significant correlation was observed between the number of oocytes retrieved following COH and gestation at delivery (r = 0.089, P = 0.048). Conclusions While a woman's response to COH was weakly negatively correlated with gestation at delivery, this relationship was not of major clinical importance as the rate of preterm delivery and the median gestation at delivery were not significantly different between pregnancies resulting from fresh or FETs in our study cohort.