Abstract
STUDY QUESTION: Is first-trimester intrauterine growth different between pregnancies resulting in live births established using fresh or frozen single embryo transfers (ET), and is it related to the embryo stage at the timing of transfer? SUMMARY ANSWER: Among pregnancies resulting in live birth, the use of frozen embryo transfer was associated with larger crown-rump length in the first trimester compared with fresh embryo transfer, regardless of the embryo stage at the timing of transfer. WHAT IS KNOWN ALREADY: Studies have indicated that singletons born following frozen/thawed ET have a higher likelihood of increased birthweight, in comparison with children born after fresh ET. A few studies have also suggested greater crown-rump length (CRL) in pregnancies after frozen embryo transfer (FET) compared to fresh ET. However, discrepancies exist regarding the intrauterine growth between fresh ET and FET groups, and the knowledge regarding the effect of embryo stage at transfer (cleavage stage vs. blastocyst stage) on the first-trimester growth remains limited. STUDY DESIGN SIZE DURATION: This prospective cohort study included all single ETs resulting in singleton pregnancies and live births following either FET or fresh ET at an academic reproductive medical center during 2013-2024. PARTICIPANTS/MATERIALS SETTING METHODS: Among 3445 singleton live births during the study period, 54.3% were after FET, and 62.2% after blastocyst transfer. All underwent an ultrasound scan between 6 and 12 gestational weeks to measure the CRL (mm). Generalized additive models were used to compare the CRL between pregnancies after FET and fresh ET, as well as to investigate associations according to embryo stage at transfer (cleavage stage or blastocyst stage), accounting for multiple confounders. MAIN RESULTS AND THE ROLE OF CHANCE: Compared with pregnancies after fresh ET, the CRL in pregnancies after FET was larger after adjusting for gestational age, embryo stage at transfer, parental age, maternal BMI, and smoking status in the first trimester, though the difference was small (β = 0.30 mm, 95% CI: 0.00-0.60, P = 0.053). The difference remained in subgroups defined by the embryo stage at the timing of transfer (β = 0.31 mm, 95% CI: -0.01-0.62, P = 0.055 and β = 0.22 mm, 95% CI: 0.06-0.39, P = 0.008 for cleavage and blastocyst stage, respectively). Normalizing CRL to a reference population conceived naturally from the INTERGROWTH-21(st) project showed that the mean CRL Z-scores in pregnancies after fresh ET and FET were both greater than 0 before day 63, suggesting the CRL in the present study population was larger than that of the non-ART population. LIMITATIONS REASONS FOR CAUTION: The lack of adjustment for other relevant confounders, such as maternal infertility factors, could lead to unmeasured confounding. Additionally, the predominance of fresh embryos in the cleavage-stage ET group limited statistical power. WIDER IMPLICATIONS OF THE FINDINGS: A difference in the intrauterine growth of singletons born after frozen and fresh ETs appears early. The effect of FET on the CRL is similar regardless of whether cleavage-stage embryo or blastocyst transfer is used. Thus, the selection of embryo stage at the timing of transfer needs to be informed by other clinical factors. Besides, reliance on CRL for redating pregnancies conceived by assisted reproductive technologies warrants caution as it may overestimate the gestational age and miss potential growth restriction. STUDY FUNDING/COMPETING INTERESTS: This study was supported by the Swedish Childhood Cancer Fund, Swedish Cancer Society, Radiumhemmets Research Funds, The Swedish Research Council, ALF Grants from Region Stockholm, and Karolinska Institutet research grants to KARW. ARP is supported by fundings of the Beatriz Galindo Program BG23/00015 with further support from FEDER and UE (PID2024-160756OA-I00) both from the Spanish Ministry of Science, Innovation and Universities and KI Research Foundation Grants 2024-2025(2024-02566). The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication. The authors declare no conflicts of interest regarding this work. TRIAL REGISTRATION NUMBER: N/A.