Twins can occur in two ways. In around a third of cases just one egg is fertilised but within days it splits, each half developing into an identical baby. In the remaining two thirds, two eggs are released at the same time and both fertilised by separate sperm, resulting in two non-identical babies.

Non-identical twins (also called fraternal, dizgotic or binovular) are no more alike than any other brothers or sisters. These twins are conceived at the same time or one following the other in a single menstrual cycle – so non-identical twins can even have different fathers! They share on average, just half their genes, so they can look very like each other or totally different.
Identical twins (also called monozygotic or uniovular) occur when for no reason the fertilised egg splits during the first 14 days and each twin gets the same genetic make-up. More identical twins have been born to mothers who have had fertility treatment with ovulation-inducing drugs.

On ultrasound a single placenta means the babies are most likely to be identical. Two placentas are inconclusive – they could be identical or fraternal.

Multiple pregnancies carry more complications than a singleton pregnancy and identical twins (sharing one placenta) are at higher risk of complications than nonidentical twins. It is important therefore to establish whether there is one or two placentas at the first ultrasound scan and a management plan established.

Complications of a Twin Pregnancy
Carrying more than one baby can put an extra strain on your body’s resources with many of the usual problems of pregnancy starting sooner or being exaggerated.
Nausea may be more bothersome because of the higher level of circulating hormones. Heartburn, indigestion, and the need to empty the bladder are more likely as the enlarging uterus presses on other organs. Sleep problems may also occur.
More serious complications are also possible. Threatened miscarriage occurs more often, as does vaginal bleeding, perhaps partly because one or both placentas are lying low in the uterus.
In rare cases one twin dies in the womb in the very early weeks leaving the surviving twin. An USS will reveal the empty sac which is reabsorbed back into the body. This is called the “vanishing twin” syndrome
Growth rate of the twins is monitored frequently by USS to check for intrauterine growth retardation.
Premature labour is common in twins and therefore they may be more vulnerable to breathing problems, infection and feeding problems according to their prematurity. Some twins may require time in a special care baby unit.
High blood pressure and pre-eclampsia are a risk with twin pregnancy and may require early delivery.
Rest is important as well as a healthy diet rich in iron. An iron supplement should be taken.
Identical twins may develop a rare complication known as “twin to twin transfusion” (TTTS).
It involves the transference of blood from one twin directly to the other twin in the womb and results in the donor twin (or twin 1) born smaller and the recipient twin (or twin 2) born larger, and with too much blood volume. In severe cases the donor twin may need a blood transfusion and the recipient twin may need blood volume reduction. If the “twin to twin transfusion” is mild full recovery is expected for both babies.

DELIVERY
If both twins are head down, a normal vaginal delivery is possible, and even if one twin is breech, he may be able to be turned to be born head down. Otherwise, or if the first baby is breech a Caesarean Section may be required.
During labour both babies are continuously monitored.
An epidural anaesthetic is usually a good choice for pain relief.