Effect of age on decisions about the numbers of embryos to transfer in assisted conception: a prospective study

Background Elective single-embryo transfer has been proposed as a strategy to reduce the risk of multiple birth and adverse pregnancy outcomes after in-vitro fertilisation (IVF). Whether this approach should be restricted to young women is unclear. Methods In a prospective study of UK Human Fertilisation and Embryology Authority data, we investigated whether perinatal livebirth outcomes varied by the number of embryos transferred in relation to maternal age. We compared rates of livebirth, multiple births, low birthweight (<2.5 kg), preterm birth (<37 weeks), and severe preterm birth (<33 weeks) in women younger than 40 years and those aged 40 years or older. We used logistic and binomial regression methods to assess, respectively, relative risk and absolute differences in risk. Findings We assessed 124 148 IVF cycles overall, which yielded 33 514 livebirths. The odds ratios of livebirth were higher in women aged 40 years or older than in those younger than 40 years when two embryos were transferred compared with one embryo (3.12, 95% CI 2.56-3.77 vs 2.33, 2.20-2.46; p=0.0006 for interaction), but the absolute difference in risk of livebirth was smaller (0.090, 0.080-0.099 for women >= 40 years vs 0.156, 0.148-0.163 for those <40 years; p<0.0001). The odds ratios and absolute risk differences for multiple birth, preterm birth, and low birthweight were all smaller in older than in younger women (analyses were done in 32 732 cycles in which a livebirth had resulted and data on gestational age and birthweight were complete). Livebirth rates did not increase with transfer of three embryos, but the risk of adverse perinatal outcomes did increase. Interpretation Transfer of three or more embryos at any age should be avoided. The decision to transfer one or two embryos should be based on prognostic indicators, such as age.