Twin births have always been part of nature but with rising twin rates in modern times IVF has a lot to answer for.
Identical or “monozygotic” twins, (where a single egg and sperm combine to form an early embryo, which goes on to “split”, forming two babies) occur in about 4 in 1000 births. The rate of this happening is pretty consistent across all populations and races. This suggests an embryo splitting is a random event and isn’t influenced by your genes.
In contrast, fraternal, dizygotic or “non-identical” twin birth (where two eggs are simultaneously released and fertilized by two separate sperm cells to form two genetically distinct embryos) can have varying incidence in different racial groups. Sometimes a tendency to this trait can run in families.
For example in Caucasian populations, dizygotic (two egg) twins occur in 8 in 1000 births. In African populations, twins are twice as common (16 in 1000 births), whereas in Asian populations it’s only 4 in 1000 births affected.
In Australia, overall, about 1 in 80 births are twins so how does this compute?
The simple answer is IVF and the process of double embryo transfer, or simply put, putting back two embryos at the same time.
The idea of twins is pretty gorgeous and most couples wrestling with infertility at some point idealize the concept of an “instant family”, “job done” or “two for the price of one”. But IVF experts now agree on the most part that single embryo transfer should in fact be gold standard.
So why shouldn’t aiming for twins be the IVF practice norm?
Twin pregnancies are by definition higher risk affairs. Mothers face increased risks of high blood pressure and pre-eclampsia, gestational diabetes and operative deliveries (c-sections) while babies face higher risk of growth restriction (due to an overtaxed placenta and limited physical room) and premature delivery. It is preterm birth that carries a lot of danger for twins, and the earlier they are born, the more likely they are to suffer developmental injuries. Issues can range from problems with vision and hearing to respiratory and cognitive developmental problems, cerebral palsy and learning disabilities.
Humans are definitely best designed to have one baby at a time. So why would an IVF doctor put back two embryos?
Before we got really good at growing and freezing embryos in the IVF lab, doctors considered the best thing we could do by our patients was to perform double embryo transfers (to improve the chance of conceiving with IVF in a given month). We used to accept as a trade off, an increased twin rate affecting up to 50% of IVF pregnancies. In more modern times, with thanks to vitrification freezing technology, in the best IVF units (such as at Melbourne IVF), pregnancies are now equally as likely to result from frozen embryo transfers. This fact gives me confidence that by freezing your additional embryos (rather than transferring them at once), we are not compromising your ultimate chance of having a baby.
So did Beyonce and Amal have IVF? Maybe. But isn’t the real question “so what”? In Australia today there is statistically at least one IVF baby in every classroom and, in my opinion, overcoming infertility is a tremendous thing.