Multiple pregnancy (twins)
Multiple pregnancies account for 1.6% of pregnancies in Australia, and about 98% of these are twins.
Two-thirds of twins result from "double ovulation", or the release of two eggs from a woman's ovaries, and are therefore non-identical; they are no closer related than any other siblings from the same parents. The other one-third result from "splitting" of an egg after fertilisation, and these twins therefore are genetically "identical" (or, in obstetric parlance, "monozygotic").
The chance of woman having twins increases with her age, her height, her number of previous pregnancies, if a first-degree relative (sister or mother) has had twins and, of course, many fertility treatments. Twins are also more common among women of African ethnicity, and less common among women of Asian ethnicity.
Interestingly, the rate of identical twinning (1 in 250 pregnancies) has remained stable since the time of Cain & Abel (incidentally, not twins...), and is completely uninfluenced by the aforementioned associations.
As with all pregnancies, most twin pregnancies are uncomplicated, and pregnancy and delivery progress smoothly. Compared with singleton pregnancies, however, twin pregnancy is considered "high risk", as essentially all complications of pregnancy occur more frequently in twins - the notable exception being overdue pregnancy! These include preterm birth, pre-eclampsia and gestational diabetes, among others.
A common misconception (no pun intended) is that twins with one placenta are identical and those with two placentas are non-identical. Well, this is half-right. All twins who share a placenta will be identical; however, having one placenta each does not guarantee your twins are non-identical (unless one is a boy and the other a girl!). To be a little more precise, if each baby in a twin pair has a placenta, there's a 14% chance that they're identical; if they turn out to be the same gender, there's a 25% chance they're identical. Beyond knowing whether to buy matching onesies, this little fact begins to weigh in during discussions about prenatal screening and testing.
As obstetricians, however, we do tend to fixate a little on the number of placentas, because this helps us determine just how high- or low(er)- risk a twin pregnancy may be, and how frequently we need to monitor the babies' development. If you are pregnant with twins, you will require more frequent ultrasounds to monitor each baby's growth and to watch out for early signs of complications. Twins are generally delivered earlier than singleton pregnancies (usually somewhere between 37 and 38 weeks, depending on the clinical situation), sometimes by elective caesarean section. Vaginal twin birth is, however, a safe and acceptable option for women, providing the conditions of their particular pregnancy are favourable. I have trained in, and developed the specialised skills required for, vaginal twin birth, and am strongly supportive of a woman's choice for this mode of birth. A large, multinational randomised trial of over 2800 mums with twins has recently been published; the researchers found no difference in outcomes for mums and babies between those planning a caesarean section and those planning a vaginal birth (abstract available here). If you're expecting twins, I will discuss with you at length all options for delivery (and associated risks and benefits) in your particular case.
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David is a Fellow of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists, and undertook his specialist training in Queensland. He is highly skilled in the management of complex and high-risk pregnancies, and has special training in minimally-invasive surgery, endometriosis, pelvic floor and incontinence surgery. David has completed a Master of Reproductive Medicine and is skilled in the assessment and management of fertility problems, and can offer the full range of assisted reproductive treatments. He is a Senior Lecturer with The University of Queensland Medical School, and has published both medical journal and textbook contributions.
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