Association between the number of oocytes retrieved and neonatal outcomes after freeze-all IVF cycles
Authors: Yang Xiaoyan , Zhang Jie , Wu Jiayi , Huang Jiaan , Chen Qiuju , Lu Xuefeng , Lyu Qifeng , Kuang Yanping , Wang Yun 
Published: September 2019

Abstract

Study question
Is there any association between the number of oocytes retrieved and neonatal outcomes following IVF/ICSI treatment for patients using a freeze-all strategy?

Summary answer
There was no increased risk of adverse neonatal outcomes in cycles with high number of oocytes retrieved (≥ 16) compared to those with 10–15 oocytes retrieved in freeze-all cycles.

What is known already
Recent studies have found that there is an increased risk of preterm birth (PTB, <37 weeks gestation) and low birth weight (LBW, <2500 g) following IVF in women with a high number (>20) of oocytes retrieved in fresh embryo transfer (ET) cycles. Other studies have found that there is an association between the number of oocytes retrieved and placenta praevia. However, the association between the number of oocytes retrieved and neonatal outcomes when using a freeze-all strategy is unknown.

Study design, size, duration
This retrospective cohort study included 14 170 women with singleton deliveries achieved by a freeze-all strategy performed between November 2006 and December 2017 in China. Only the first delivery from one episode of ovarian stimulation was included.

Participants/materials, setting, methods
Only cycles using a freeze-all strategy performed during the study period and resulting in singleton live births were included. Patients were categorized into five groups according to the number of oocytes retrieved: 1–3, 4–9, 10–15, 16–20 or >20 oocytes. In univariate and multivariate logistic regression analysis of the association between ovarian response and the outcomes of PTB, early PTB, LBW and other neonatal outcomes, the 10 to 15 oocyte category was used as a reference and other four groups were analysed as dummy variables. Multiple linear regression analysis was used to evaluate possible associations of birth weight z-scores and the number of oocytes retrieved (analysed as a continuous variable) with other confounding factors.

Main results and the role of chance
After adjusted for confounding factors, no significant differences were observed in the risk of PTB (P = 0.837), LBW (P = 0.974), early PTB (P = 0.341), very LBW (P = 0.848), congenital malformation (P = 0.916) and other adverse neonatal outcome among patients with different number of oocytes retrieved. There was a higher risk of early PTB among women with a poor ovarian response (1–3 oocytes) compared with women with a normal response (10–15 oocytes) (1.5% vs 0.8%), crude odds ratio (OR): 2.001, 95% CI: 1.159–3.465, P = 0.013. However, the difference was not significant after adjusting for confounders, adjusted OR: 1.753, 95% CI: 0.997–3.081, P = 0.051.

Limitations, reasons for caution
Data on some known confounders such as smoking and medical history of gestational diabetes mellitus and preeclampsia were lacking. As with any retrospective study, unknown confounders may affect outcomes.

Wider implications of the findings
In the freeze-all cycles, there was no association between number of oocytes retrieved and adverse neonatal outcomes. This is a reassuring finding for both clinicians and patients who are planning to use freeze-all cycles for a variety of indications.

Study funding/competing interest(S)
Grants from the National Natural Science Foundation of China (NSFC) (31770989 to Y.W.) and the Shanghai Ninth People’s Hospital Foundation of China (JYLJ030 to Y.W.). None of the authors have any conflicts of interest to declare.