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Pregnancy, Fertility Dr Erin Nesbitt-Hawes Director
Two little jellybeans and two little heartbeats in the fuzzy haze of the ultrasound… Twins! How do twins come about?

Twins used to be a rare fascination in the schoolyard however they are becoming increasingly common due to a number of factors. In 2012 1.4% of births were twins. To understand more we should start at the beginning – how does a twin pregnancy occur?

There are a number of different types of twin pregnancy:

Dizygotic twins (non-identical or “fraternal” twins) come from two separately fertilised eggs with two individual sperm. Each baby has its own placenta and amniotic sac. These are the most common type of twin pregnancy (70%) and tend to run in families. As these twins are a result of two eggs being released, only a family history on the mother’s side is relevant. They may also occur as a result of advanced maternal age or assisted fertility techniques such as the use of clomiphene citrate (a medication to stimulate ovulation) or IVF (in vitro fertilisation) if multiple embryos are transferred. Race is another factor that may increase your likelihood of conceiving twins, being highest in those of African descent and lowest in those of Asian descent.

Monozygotic twins (identical twins) occur when a fertilised egg divides in the first 1-2 weeks forming two separate embryos. Babies share a placenta and may have individual or a shared amniotic sac. Because they originate from the same fertilised egg, these babies share the same genetic material and are always the same sex. These twins are less common and occur randomly without any family history. It is not clear why, but assisted reproductive techniques also increase the likelihood of identical twins.

What about triplets?

Triplets may be formed from one, two or three fertilised eggs and as a result may be all identical, one identical twin pair and one non-identical baby or three non-identical babies. They are much less common than twin pregnancies.

Twin (and other multiple) pregnancies are considered high-risk pregnancies. As well as discomforts associated with carrying multiple babies (back aches, pelvic pain, increased swelling), there is a tendency to develop many of the complications of pregnancy such as high blood pressure (preeclampsia) and diabetes.  The babies are also at higher risk of being smaller than expected and premature and may need to go to neonatal intensive care for a period after birth. Indeed a dizygotic twin pregnancy has an average gestation period of 38 weeks (compared to the 40 weeks of a singleton pregnancy). Monozygotic twins are even higher risk again and need to be closely monitored during the pregnancy. A complication called twin-twin transfusion syndrome (TTTS) may occur due to the shared placenta and can be life threatening. Overall, if you have a multiple pregnancy you can be assured of frequent visits to your obstetrician and many ultrasound scans!

How are twins delivered?

Twin pregnancies may be delivered vaginally or via a caesarean. Factors that are important in this decision include the type of twin pregnancy (if there is a shared placenta and amniotic sac a caesarean will be recommended), the way the first twin is facing (head down is best), the size of the twins (particularly in relation to each other) and whether the mother has had a caesarean delivery previously. When planning for a vaginal delivery an epidural may be recommended in case of complications with delivery of the second twin.  At Alana Healthcare for Women, your obstetrician will discuss the mode of delivery with you in order to come to a shared decision on what will be best for you and your babies.