Many families undergoing IVF assume that transferring just one embryo will result in only one baby. Yet a clinical surprise can occur: after transferring a single embryo, ultrasound detects two fetal heartbeats. This happens when the early embryo spontaneously splits to form identical twins.
One clinical case illustrates this scenario: a couple underwent IVF, with one healthy embryo transferred. Four weeks later, ultrasound revealed two heartbeats instead of one. The embryo had naturally split in the early pre‑ and post‑implantation stage, producing identical twins with matching genetic material. Although the babies were born prematurely at just over 31 weeks with low birth weight and required care in the neonatal intensive care unit, they recovered well and were discharged home healthy. This case highlights an important reality for people pursuing assisted reproduction: single‑embryo transfer does not guarantee a singleton pregnancy.
The High‑Risk Nature of Twin Pregnancy
Whether twins result from transferring two embryos or from the splitting of one embryo, twin pregnancy is classified as high‑risk. Clinical data shows around 60% of twin pregnancies end in preterm birth (delivery before 37 weeks gestation). The earlier the preterm delivery, the higher the risk of neonatal complications including respiratory distress, feeding difficulties and infection.Key risks of twin pregnancy:
Preterm birth and low birth weight; more than half of twins are born with low birth weight.
Significantly elevated risks of maternal gestational diabetes, pre‑eclampsia and caesarean section.
Higher likelihood of admission to the neonatal intensive care unit (NICU) and longer hospital stays for newborns.
Even when outcomes are positive, twin pregnancy carries substantial risks for both mother and babies, and should not be viewed simply as “double joy”.
Single‑Embryo Transfer: Mainstream Practice Yet Still Carries Splitting Risk
To avoid the hazards of multiple pregnancy, elective single‑embryo transfer (eSET) has become the standard of care in reproductive medicine worldwide. According to US CDC data, between 2013 and 2022, the proportion of IVF cycles using single‑embryo transfer rose from 31% to 86%.
Nevertheless, spontaneous embryo splitting remains possible even with single‑embryo transfer. Medical literature reports that the rate of a single embryo splitting into identical twins in assisted‑reproduction cycles is approximately 1‑2%, higher than the rate of identical twins in natural conception. This is a random developmental event that cannot be predicted or controlled, and is an objective fact all IVF families should understand.
Frequently Asked Questions
Q1: Can one transferred IVF embryo split into twins? A: Yes, this is a sporadic occurrence. A single early embryo spontaneously divides into two fully‑developed cell masses, producing genetically identical identical twins. The incidence in assisted reproduction is about 1‑2%.
Q2: What risks come with twin pregnancy after IVF? A: Compared with singleton pregnancy, twin pregnancy increases maternal risks of gestational hypertension, gestational diabetes and caesarean section. It also raises the risk of preterm birth, low birth weight and NICU admission for infants. Twin pregnancies require high‑risk obstetric management throughout gestation.
Q3: Why do reproductive clinicians generally recommend single‑embryo transfer? A: The primary goal is to reduce maternal‑fetal risks associated with multiple pregnancy, prioritising maternal safety and newborn health, while maintaining good cumulative live‑birth rates. It is recommended by reproductive guidelines both domestically and internationally.
Q4: How widespread is single‑embryo transfer today? A: Overseas statistics show a sharp rise in single‑embryo transfer rates. China’s Expert Consensus on Embryo Transfer Number also prioritises single‑embryo transfer for eligible patients to control multiple‑pregnancy risks.
Closing Remarks
Assisted reproductive technology helps families fulfil their fertility goals, yet embryonic development still contains unpredictable natural variables. Single‑embryo transfer greatly reduces multiple‑pregnancy risk but cannot completely rule out the small‑probability event of monozygotic twinning. For IVF families, the priority of childbearing should be maternal and infant safety, rather than pursuing twins.
