Creating multiples

Yay, you’re pregnant! But for some expecting mothers, there will be another surprise in store – more than one heartbeat on the scan. We explain just how multiples come to be, writes Julia Boltt

HOW DOES IT HAPPEN?

There are two types of twins, distinguished by the way the foetuses form. Identical twins (also called paternal twins, or referred to as monozygotic) form after the division of a single fertilised egg. Their genetic material is therefore exactly the same and, as the name suggests, they typically look identical

or extremely similar to each other. Identical twins share a placenta, but each has their own umbilical cord attaching them to the placenta. Fraternal twins (dizygotic) occur when two separate fertilised eggs implant in the same uterine cavity. These twins therefore have different genetic compositions and aren’t identical in appearance. They will share about 50 percent of their DNA, just like any other siblings, and as a result can look quite similar or quite different from each other. Fraternal twins may be two babies of the same sex, or they may be a boy/ girl pairing.

Most twins are fraternal and are more commonly a same-sex pairing (both boys or both girls) than a boy/girl set. “Dizygotic (fraternal) twins are more common than monozygotic (identical) twins, approximately 70 and 30 percent of twins respectively, excluding the use of assisted reproductive technology,” says Dr Marlene Bothma, specialist obstetrician and gynaecologist with a special interest in multiple pregnancies.

WHAT ARE THE CHANCES?

Identical twins are a random occurrence, and the causes are unknown – there’s no hereditary trait. Even in families with a high incidence of identical twins, it’s down to chance.

Fraternal twins occur when more than one egg is released in a cycle, and may run in families, as hyperovulation (releasing more than one egg in a cycle) may be passed from mother to daughter. Men can also carry the trait for hyperovulation, but it doesn’t increase his chances of having twins with his partner – he will pass this onto his daughter, who would have a greater chance of having twins.

“Other factors that increase your chances of having fraternal twins include the increasing age of the mother, ovulation induction, in-vitro fertilisation (IVF), being tall and originating from certain countries (particularly African countries),” explains Dr Jana Rossouw, a specialist in gynaecology and obstetrics at Tygerberg Hospital in Cape Town.

TWO’S COMPANY, THREE’S A CROWD

Most triplets are trizygotic – meaning each foetus forms from a separately fertilised egg. Triplets may consist of an identical pair of twins and a singleton all growing in the same uterus. Identical triplets are extremely rare and happen when the original egg splits, and then one of the resultant cells splits again. Triplets can be a natural occurrence, but “the rate of triplet and higher-order multiple births is higher than the naturally occurring rate due to medically assisted conception, particularly IVF and controlled ovarian hyperstimulation (fertility treatments), in countries where this technology is widely available,” Dr Bothma explains. Increasing maternal age at conception has also contributed to the increased rate of triplet pregnancies.

YES, THERE ARE RISKS

Understandably, carrying multiples puts a lot more strain on your body than a singleton pregnancy.

“A multiple pregnancy carries a higher risk throughout the pregnancy for mom, babies and placental function,” says Dr Rossouw. Maternal risks include a higher risk of exaggerated symptoms (morning sickness or hyperemesis gravidarum) in the first trimester, hypertensive disorders like gestational hypertension and pre-eclampsia, gestational diabetes, pre-term labour and an increased likelihood of needing a caesarean section, as well as thromboembolism (blood clots in pregnancy), and postpartum haemorrhage (excessive bleeding at birth).

“Foetal risks include an increased risk of miscarriage, a higher chance of prematurity, and possible growth restriction. Placental problems leading to foetal growth restriction are also more common, as well as the serious complication of abruptio placentae – the premature separation of the placenta from the uterus,” says Dr Rossouw. “Identical twins who share a placenta may be at risk for twin-twin transfusion syndrome, imbalances in foeto-placental blood flow in the shared placental circulation. Another possible complication is selective foetal growth restriction, a discordance in the growth between the two twins, when there is one significantly smaller twin or a discordance in weight between the two twins of more than 25 percent,” explains Dr Bothma.

The death of one twin of a monochorionic pair (identical twins who share a placenta) can cause morbidity or mortality in the co-twin due to their shared placental circulation, she says, explaining that monochorionic twins have a higher rate of congenital anomalies than dichorionic twins and singletons. For monoamniotic twins (sharing an amniotic sac), intertwin cord entanglement and conjoined twins are foetal complications of particular concern, says Dr Bothma.

GET READY…

Twins generally deliver earlier, be that due to a medical reason or spontaneous pre-term labour. “The average gestational age at delivery is 35 weeks for twins, 32 weeks for triplets and 29 weeks for quadruplets. As a result, 25 percent of twins and 75 percent of triplets require admission to the neonatal intensive care unit (NICU),” explains Dr Bothma.

As for natural birth versus a caesarean delivery for a mom expecting multiples, “It depends on the position of the babies in her uterus, her health, and her babies’ health,” says Dr Bothma. “If she and her babies are healthy, and the first baby is coming head first, she might be able to deliver her babies vaginally. If not, it will most likely be safer to deliver by caesarean section. Each multiple pregnancy would have to be assessed individually. We choose a delivery route based upon the health of the mother, presentation (the position of the babies) and amnionicity and chorionicity (whether they share a placenta and amniotic sac), as well as the presence or absence of standard obstetrical indications for caesarean delivery, for example, placenta praevia.”

CARRYING MULTIPLES PUTS A LOT MORE STRAIN ON YOUR BODY