Twins can have different parents. How is that possible? A fertility expert explains

A Gold Coast woman has given birth to twins who aren’t genetically related to each other, after becoming pregnant with a surrogate couple’s embryo and her own biological baby at the same time. This has produced one of the rarest outcomes in reproductive medicine, the first in Australia and only the third reported worldwide. Queensland law forbids twins being separated. So the case, ruled on by the Children’s Court of Queensland, has forced the courts to confront a scenario the state’s surrogacy laws were never written to accommodate. What happened The surrogate, a 27-year-old woman, already a mother of five, had agreed to carry a baby for a commissioning couple. The intended mother was born without a uterus and could not carry a pregnancy. The couple entered an altruistic surrogacy arrangement after connecting with the potential surrogate through a mutual friend and on social media. The commissioning couple’s embryo was transferred to the surrogate through fertility treatment. Later that same month, an ultrasound delivered a surprise: she was carrying twins. She subsequently delivered a boy and a girl by caesarean section. DNA testing during early pregnancy revealed the baby girl was the biological daughter of the commissioning couple, while the boy was the biological child of the surrogate and her husband. At the same time as undergoing the embryo transfer, the surrogate had also conceived naturally with her partner. The result was a single pregnancy carrying two babies from two entirely separate families. Since the birth in November, each couple has been raising their own biological child, and there is no dispute between them over parentage. How is this possible? Let’s look at how twins ordinarily arise. In natural conception, there are two kinds: identical (monozygotic) twins occur when a single embryo splits after fertilisation into two separate embryos. Because they come from one egg and one sperm, they are genetically the same and are therefore always the same sex. fraternal (dizygotic) twins occur when two separate eggs are released during ovulation and fertilised by two different sperm in the same cycle. Genetically they are no more alike than any other siblings, and they may be the same sex or different sexes. Read more: Curious Kids: why are some twins identical and some not? Rarer still is a phenomenon called superfecundation, in which two eggs released in the same cycle are fertilised during separate acts of intercourse — sometimes involving two different fathers. This phenomenon is called heteropaternal superfecundation. The resulting twins are fraternal, but with genetically distinct paternal origins. The Queensland case is a striking variation on this theme: not two fathers through intercourse, but one pregnancy combining an IVF embryo transfer and a natural conception in the same ovulation window. How surrogacy can produce twins In IVF surrogacy, an embryo is created from an egg and sperm of the commissioning couple. It’s grown in the laboratory to around day five and typically frozen before being transferred into the surrogate’s uterus sometime later. Clinics use various techniques to encourage the embryo to implant. One is to time the transfer to the surrogate’s natural ovulation. The aim is to “persuade” the surrogate’s body that the embryo is the product of a natural cycle, improving the odds of a successful pregnancy. But timing the transfer to natural ovulation carries an obvious risk. Because the surrogate is genuinely fertile at that moment, a natural pregnancy is entirely possible. Surrogates are usually advised to avoid intercourse around the time of ovulation and embryo transfer. In this case, both an IVF embryo and a naturally conceived embryo took hold, producing a twin pregnancy in which the two babies were biologically unrelated. The ethical and legal implications The rarity of the situation collided directly with Queensland law. The state’s surrogacy legislation is designed to prevent the separation of “birth siblings” where a surrogacy results in multiple births. This protection is intended to stop twins being split up. But this would have blocked the two families from raising their respective biological children apart. Judge Jodie Wooldridge KC resolved the tension by finding the twins were not “birth siblings” as defined in section 24 of the Surrogacy Act. Because the two children were conceived separately and were not genetically related, they were not the product of the same pregnancy in the sense the law intended. So the provision preventing separation did not apply. That finding formalised each biological couple’s parentage and allowed the children to be raised by their respective families. The children’s best interests remained the paramount consideration. An independent counsellor who assessed both families concluded that separation from birth was unlikely to cause psychological harm on its own. Rather, their outcomes would depend far more on the quality of caregiving and openness about the children’s origins. By this point, the families had an established relationship, and each couple hoped the children would grow up aware of each other and their rather unorthodox connection.