Sunanda is exhilarated and at the same time nervous. She has had an ultrasound scan and discovered she is having twins! When she saw the two babies inside her, her heart skipped a beat and then started racing. Her husband too is thrilled at this unexpected bonus!

Sheela and her husband have been trying for a pregnancy for a few years. They had undergone treatment with medications and have now conceived after a fertility procedure. She had her pregnancy confirmed after an ultrasound scan. She too is having twins.

How common is a twin pregnancy?

The chance of having identical twins, i.e. twins from a single egg, has remained fairly constant throughout the world. One out of 250 women will have identical twins. However, the rate of having non-identical twins or fraternal twins, i.e. twins formed from two different eggs, is increasing all over.

The advent of IVF (in vitro fertilisation or ‘test tube baby’) has also contributed to the increased rate of twins. This procedure typically involves implanting more than one embryo in the uterus and, therefore, is more likely to result in twins, triplets, quadruplets or more.

How are twins formed?

There are two different kinds of twins:

Identical twins are rarer and are derived from a single fertilised egg. Identical twins may share a placenta, but each baby usually has its own amniotic sac. The twins will have identical genes and be of the same sex, have the same blood type, hair colour and eye colour, and are difficult to differentiate in appearance.

Non-identical twins or fraternal twins are derived from two fertilised eggs released during the same menstrual cycle. These twins will have genetic differences in the same way as normal brothers and sisters. They may or may not be of the same sex.

Diagnosing a twin pregnancy

There are certain situations when your obstetrician may suspect twins:

if there is a history of twins in your family

if you have had twins in an earlier pregnancy

if you have had treatment for infertility

if your uterus appears unusually large

A suspected twin pregnancy is confirmed with an ultrasound scan.

What to expect with a twin pregnancy

Having more than one baby means that you may have more of everything: more side effects, more frequent check-ups, more nutrition, more rest. Due to the increased volume of the uterus, you might find it uncomfortable to move around. It is nevertheless important to be active and keep up a gentle exercise regimen.

Complications for the mother

Premature or preterm labour: In a twin pregnancy, it is not unusual to go into labour between 36 and 38 weeks. You might be asked to be at rest and to decrease excessive exertion from the 36th week of pregnancy.

Premature birth: If preterm labour cannot be prevented or stopped once it has started, the babies may be born too early. Premature or preterm birth is one of the commonest risks for twins.

High blood pressure: A woman carrying twins has a greater risk of developing high blood pressure or preeclampsia. Preeclampsia is a condition of pregnancy in which there is high blood pressure and protein (albumin) in the urine.

Gestational diabetes: Mothers of twins are more likely to develop gestational diabetes, a type of diabetes that occurs only during pregnancy. Gestational diabetes may damage the placenta and increase the risk of the babies developing breathing problems at birth.

Growth of twins

Twins are more likely to be smaller than average. Ultrasound scans are done more often in a twin pregnancy mainly to monitor the growth of the babies. Twins are said to have discordant growth if one is much smaller than the other. The amount of amniotic fluid surrounding each baby also needs to be monitored.

Delivery of twins

Most twins can be delivered vaginally, if the lower baby is coming down head first. A vaginal birth may also be possible when the lower twin is in the head-down position but the second twin is not. Once the first twin is born, the other twin can sometimes be turned or delivered with feet or buttocks first. When the first twin is not in the head-down position, both twins are delivered by caesarean birth.

(The author is a Chennai-based obstetrician and gynaecologist with a special interest in women’s health issues.)