Vitrification does not affect birth weight: lessons from the oocyte donation model

Research question: Is embryo cryopreservation a cause of high birth weight and large for gestational age (LGA) in singletons resulting from vitrified-warmed embryo transfer? Design: Retrospective cohort study evaluating 670 oocyte recipients who underwent fresh (367 cycles) or vitrified-warmed embryo transfer (303 cycles) at Instituto Bernabeu between July 2017 and March 2019. All single blastocyst transfers carried out in an artificial cycle that resulted in a singleton live birth were included. Results: Maternal age (42.21 +/- 4.45; 42.79 +/- 3.83; P = 0.519), body mass index (23.34 +/- 3.69; 23.80 +/- 3.78; P = 0.075), gestational age (38.96 +/- 1.97; 38.77 +/- 2.15; P = 0.207), maternal smoking (10.8%; 13.0%; P = 0.475), gestational diabetes (4.9%; 4.3% P = 0.854), preeclampsia (2.7%; 5.6%; P = 0.074), hypertensive disorders (3.3%; 2.3%; P = 0.494), maternal parity (multiparous 18.5%; 14.5%; P = 0.177) and liveborn gender (female 44.5%; 48.8%; P = 0.276) were not significantly different between fresh or vitrified-warmed groups. Endometrial thickness was significantly higher in the fresh versus vitrified-warmed group (8.83 +/- 1.73 versus 8.57 +/- 1.59; P = 0.035, respectively). Oocyte donor height was similar between the fresh versus vitrified-warmed group (163.22 +/- 5.88 versus 164.27 +/- 6.66 cm; P = 0.057, respectively). Mean birth weight was not significantly different (3239.21 +/- 550.43; 3224.56 +/- 570.83; adjusted P = 0.058). No differences were observed in macrosomia (7.1%; 6.3%; adjusted OR 0.857, 95% CI 0.314 to 2.340, P = 0.764), LGA (6.0%; 6.7%; adjusted OR 0.450, 95% CI 0.176 to 1.149, P = 0.095), pre-term birth (10.9%; 9.0% adjusted P = 0.997), very pre-term birth (0.8%; 1.3%; adjusted P = 1.000), extremely pre-term birth (0%; 1.0%; adjusted P = 0.998); underweight (10.0%; 70%; adjusted P = 0.050); very low weight (0.6; 1.1%; adjusted P = 1.000) and small for gestational age (1.9%; 0.7%; adjusted P = 0.974) between fresh or vitrified-warmed groups. Conclusion: This study eliminates potential confounders that might influence fetal growth and demonstrates that embryo vitrification and warming procedures do not affect birth weight.