{'fraction_non_alphanumeric': 0.027962347729789592, 'fraction_numerical': 0.020487264673311186, 'mean_word_length': 4.725832012678288, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Major congenital anomalies in children born after frozen embryo transfer: a cohort study 1995-2006.

STUDY QUESTION
Is there a different risk for major congenital anomalies (CAs) in children born after frozen-thawed embryo transfer (FET) compared with children born after fresh embryo transfer (ET)?


SUMMARY ANSWER
Children born after FET have a similar risk of developing major CAs as children born after fresh ET.


WHAT IS KNOWN ALREADY
The perinatal outcome in children born after FET is as good as that after fresh ET. Children born as a result of assisted reproductive technology (ART) have an increased risk for CAs when compared with spontaneously conceived children, but the knowledge on the risk for CAs in specific organ systems of children born after FET is limited.


STUDY DESIGN, SIZE, DURATION
This register-based cohort study includes women who have undergone ART treatments with ET leading to singleton births (n = 4772) between the years 1995 and 2006. The women were identified from the registers of the infertility clinics, and the corresponding births were matched with data from the Finnish Medical Birth Register (FMBR). The 10% random sample of women with spontaneous pregnancies from the FMBR served as the reference group (n = 31,243). The study data were linked with the Register of Congenital Malformations using the mothers' and children's personal identification numbers to get information on CAs. Furthermore, the personal identification numbers of the ART women were linked with the Register of Induced Abortions to find their selective terminations of pregnancy for severe foetal anomalies.


PARTICIPANTS, SETTING, METHODS
The study was focused on singleton births and included 1830 children born after FET, 2942 children born after fresh ET and 31 243 children born after spontaneous pregnancies. Only major CAs were analysed in keeping with European Concerted Action on Congenital Anomalies and Twins. The risk estimates for CAs were adjusted for the children's year of birth and maternal age, parity and socioeconomic status. The total prevalence of major CAs was counted, including both births and selective terminations of pregnancy for major fetal anomalies (n = 33).


MAIN RESULTS AND THE ROLE OF CHANCE
Among singletons at least one major CA was reported in 77 cases (4.2%) in the FET group, 132 cases (4.5%) in the fresh ET group and 994 cases (3.2%) in the reference group. The risk for at least one major CA of the children born after FET was not increased compared with the children born after fresh ET [adjusted odd ratio (aOR) 0.95; 0.71-1.27]. Furthermore, no increased risks according to the organ system affected were found between these two ART groups. When comparing the children born after ART (FET and fresh ET) with the reference group children, the risk of having at least one major CA was moderately increased in the ART group (aOR 1.24; 1.05-1.47).


LIMITATIONS, REASONS FOR CAUTION
Because of the study design we were neither able to examine the aetiology of infertility nor could we compare the data with a group of subfertile women to account for the effect of infertility per se on CAs.


WIDER IMPLICATIONS OF THE FINDINGS
Perinatal outcomes of FET children, including the risks for CAs, are good and comparable with outcomes of other ART children indicating that slow freezing is a safe method to use during ART treatments.


STUDY FUNDING/COMPETING INTEREST(S)
University Hospital of Oulu and Helsinki, Finland. THL covered the data linkages and the work of Annukka Ritvanen and Mika Gissler. There are no competing interests to be reported.