Background: The number of children born after frozen embryo transfer (FET) is steadily rising. However, studies on obstetric and perinatal outcomes are limited. Our primary aim was to compare the perinatal health of children born after FET and fresh embryo transfer, and to use data from children born after spontaneous conception as a reference.
Methods: In a register-based cohort study we evaluated the obstetric and perinatal outcomes of children born after FET (n = 2293), fresh embryo transfer (n = 4151) and those born after spontaneous pregnancy (reference group; n = 31 946). Data were collected from the registers of two infertility outpatient clinics, two university hospitals and the Finnish Medical Birth Register (1995-2006).
Results: After adjusting for confounding factors the FET group showed decreased risks of preterm birth [adjusted odd ratio (AOR) 0.83, 95% confidence interval (CI) 0.71-0.97], low birthweight (AOR 0.74; 0.62-0.88) and being small for gestational age (AOR 0.63; 0.49-0.83) compared with the fresh embryo transfer group. Mean birthweight was 134 g higher in the FET singletons versus the fresh embryo transfer singletons (P< 0.0001). When FET singletons were compared with the reference group, increased risks of preterm birth (AOR 1.45; 1.25-1.68) and low birthweight (AOR 1.22; 1.03-1.45) and a decreased risk of being small for gestational age (AOR 0.71; 0.54-0.92) were found. No excess of perinatal and infant mortality occurred between the groups.
Conclusions: Embryo freezing does not adversely affect perinatal outcome in terms of prematurity, low birthweight and being small for gestational age versus the fresh embryo transfer and the outcome is similar or even better, particularly regarding fetal growth. Our study, which is one of the largest on FET pregnancies, provides further evidence on the safety of FET.