Why IVF and twins are linked at all

The mechanism is simple. The NHS says IVF can increase the chance of twins because more than one embryo may be transferred. If two embryos are placed and both implant, that is a non-identical twin pregnancy. Nothing unusual has happened biologically; the pregnancy has simply started with two.

RCOG adds the wider context: multiple pregnancy is more common with fertility treatment, and more common as you get older, which is also the group most likely to be having treatment.

The regulator's position, stated plainly

The HFEA, which regulates fertility treatment in the UK, does not treat twins as a lucky outcome. It says the main risk of fertility treatment is a multiple birth, and that multiple births can seriously harm the health of both mother and babies. Its own summary is that at least half of twins are born premature and underweight, which can lead to serious and in some cases long-term health problems, and that the chance of early or late miscarriage is higher when carrying more than one baby.

That is a regulator writing about its own sector's outcomes, which is worth noticing.

What elective single embryo transfer means

The HFEA describes elective single embryo transfer, usually shortened to eSET, as transferring only one embryo to the womb instead of the two or three that used to be common. It says this considerably reduces the chance of a multiple birth and the health problems that come with it, and that any remaining embryos can be frozen for use in a later cycle if you wish.

That last clause is the part people miss. Single embryo transfer is not the same as having fewer embryos available. It is a decision about how many go back at once.

The 10% target

The HFEA has set clinics a target that less than 10% of their IVF births should be multiples, and says you can search for a clinic on its website to see how they are performing against it. This is a published, checkable figure for individual clinics, which makes it one of the few genuinely comparable things you can look up before choosing where to have treatment.

Transferring one embryo does not rule out twins

A single embryo can still split. When it does, the result is identical twins, and because of the timing of that split they will often share a placenta. Tommy's explains that if the egg splits between days four and eight the babies usually share one placenta but have their own amniotic sacs, which is the monochorionic diamniotic arrangement, and that this is the most common type of identical twin pregnancy.

That matters because monochorionic twins are the higher-risk group and are monitored far more closely. So a single embryo transfer that results in twins is not automatically the lower-risk kind of twin pregnancy.

If you are already pregnant with IVF twins

The question to get answered at your first scan is not whether the babies are identical. NICE NG137 says the risks associated with twin and triplet pregnancy are determined by chorionicity and not zygosity, and instructs teams to establish chorionicity and amnionicity at the time the twin pregnancy is detected, using the number of placental masses, the presence and thickness of amniotic membranes, and the lambda or T-sign.

Two embryos transferred and two implanting almost always gives dichorionic diamniotic twins with a placenta each. One embryo splitting usually gives monochorionic twins. But the scan, not the treatment record, is what decides how you are cared for, and NG137 says that if chorionicity cannot be determined even after referral, the pregnancy should be managed as monochorionic until proven otherwise.

The appointment schedule changes immediately

NG137 sets out how much monitoring follows. An uncomplicated dichorionic diamniotic twin pregnancy should be offered at least eight antenatal appointments with the core specialist team, with scans at around 11+2 to 14+1 weeks and then at 20, 24, 28, 32 and 36 weeks. An uncomplicated monochorionic diamniotic twin pregnancy should be offered at least eleven, with scans at 11+2 to 14+1 weeks and then at 16, 18, 20, 22, 24, 26, 28, 30, 32 and 34 weeks. Whichever you have, this is a different pregnancy from the one you were planning for.

What twins after IVF do not change

A twin pregnancy conceived through IVF is monitored exactly the same way as a twin pregnancy conceived any other way. NG137 makes no distinction based on how the pregnancy started. The scan schedule, the cervical length scan between 16 and 20 weeks, the growth monitoring intervals and the recommended timing of birth all follow from chorionicity alone.

What can differ is dating. If you have had IVF, your gestational age is known precisely from the transfer rather than estimated from a scan, though NG137 still asks teams to estimate gestational age from the largest baby in a twin or triplet pregnancy, to avoid dating from a baby with early growth pathology. Make sure your transfer date and embryo stage are recorded in your maternity notes, not only in your clinic file.

Questions worth asking your clinic

Before transfer, the useful questions are how many embryos the clinic is proposing to transfer and why, what its published multiple birth rate is against the HFEA target, and what happens to embryos that are not transferred. After a positive test with two sacs, the useful question is which type of twin pregnancy the scan shows, and who will be looking after it.

IVF and Twins