The incidence of twin pregnancy is approximately i in every 80 pregnancies in Great Britain. Some races are more likely to have twin pregnancies than others; they are more common in some African races than in this country, and less frequent among the Chinese. Twins are relatively uncommon in women who are under 20 or over 40, occurring most frequently in women who arc in their thirties, and then in women who are in their twenties. It is certainly true that the bigger the family the greater the likelihood of twins, but the incidence of twins probably has nothing to do with the number of babies a woman has actually had—it is just a simple fart that the more babies she has the more likely is she to catch up with the inevitable 1 pregnancy in every 80 that happens to be twins.

Twinning is certainly an inherited characteristic and may be inherited by both men and women, although it is more likely to be handed down by a daughter than by a son. Nevertheless, if there is twinning in either family there is a possibility of having a twin pregnancy. It tends to skip generations; it is quite common for grandmother and grandaughter to have twin pregnancies, whereas mother had a succession of single pregnancies.

The perinatal mortality for twin babies is four or five times higher than for singleton pregnancies, partly because of the high incidence of premature labour and also because of an increased incidence of other complications of pregnancy.

Types of Twinning

Twins may be identical or non-identical.

Identical Twins

Identical twins are formed when a single egg is fertilized by a single sperm which instead of proceeding to form a single baby divides into separate halves very early in its development so that it proceeds to develop into two separate babies within the same pregnancy sac. These babies will be identical, or uniovular twins since they have exactly the same genetic structure and the same chromosomes. While developing in the uterus they each have their own amniotic cavity, but there is a single placenta in which the blood of their circulations mix—although the circulation of the babies is completely separated from that of the mother. They are always the same sex; they will have the same features, the same colour hair, the same physical characteristics and exactly the same blood groups. In their relationship one twin will assume authority and become dominant, but to the outside world they will be similar and identical.

Siamese twins, or conjoined twins, are a uniovular (one egg) twin pregnancy which started to divide but in which the division was incomplete, so that as the babies developed they were not completely separated. Such babies may be joined by the head, the trunk or the limbs. Attempts to separate them are occasionally successful. The incidence of conjoined twins is very rare indeed.

Non-identical Twins

Non-identical twins develop from the fertilization of two separate eggs by-two separate sperms. The two separate pregnancies proceed side by side. They implant in the uterus almost without exception at the same time but at different sites so that each has its own placenta. There is therefore no mixing or communication of blood between the twins. They may or may not be of the same sex: half the sets of non-identical twins will be of the same sex and half will not. They will certainly not have the same characteristics and they may not be of the same blood group. These are known as dissimilar, nonidentical or binovular twins.

The difference between uniovular and binovular twins can be recognized at delivery by examination of the placenta. If two placentae are present then obviously the twins are binovular (dissimilar), but if only one placenta is present this is carefully examined to ensure that it is in fact a single placenta with a single chorion and two amniotic cavities, having the mixed circulation of a uniovular, identical twin pregnancy. Occasionally the two placentae of binovular twins lie side by side and appear to be one. Careful examination, however, shows there are two which are actually separate.


The exact cause of twin pregnancy is not known. A binovular pregnancy occurs when there is fertilization of two separate ova. An ejaculation may consist of at least three or four hundred million sperms, so the presence of two or more sperms in the region of the ovary at the time of ovulation is readily accepted. In fact, many authorities believe that several sperms must necessarily be present around the ovum before any single sperm can penetrate it. The fertilization of a single ovum by two sperms is not possible because the total number of chromosomes within such a cell would then be 23 from the ovum and 23 from each sperm; such a cell could not survive. The inheritance of twin pregnancy through the male side of the family can only depend on the division at a very early stage of a single fertilized ovum, that is, uniovular twins. The production of more than one ovum in each menstrual cycle is unusual and depends only on the female.

Modem fertility drugs have been developed to provoke ovulation and they occasionally lead to the formation of more than one ovum in each menstrual cycle, resulting in multiple pregnancies. When the exact characteristics of these drugs are better understood, the control of ovulation will be more accurately achieved and multiple pregnancies will occur less frequently.

Superfecundation is the fertilization of two separate ova at different acts of sexual intercourse. During the Middle Ages a twin pregnancy was always considered the result of superfecundation and was interpreted to mean that a woman had intercourse after she knew that she was pregnant. Such an act was considered indecent and the poor creatures were sometimes stoned to death. Although incidents of superfecundation have been proved both medically and legally, it is virtually impossible unless two separate acts of sexual intercourse occur very close together, and two ova have been released at ovulation. Once pregnancy has been established the circulating hormones automatically suppress further ovulation.

Diagnosis of Twin Pregnancy

The diagnosis of a twin pregnancy should be simple. It ought to be obvious that two babies are present; that the uterus is larger than it would be for one baby and that there are two of everything including two foetal hearts. In fact about 5 per cent of twin pregnancies reach delivery before the discovery of the second baby—usually to the equal embarrassment of the midwife and doctor as well as the mother.

The following factors may make the midwife or doctor consider the possibility of twin pregnancy.

Excessive nausea and vomiting may be associated with a multiple pregnancy, especially if the symptoms continue beyond the 13th or 14th week.

The uterus is consistently larger than the dates suggest. Pelvic examination is performed at the first visit to the doctor or the antenatal clinic and the presence of any fibroids or tumours is noted. If the uterus is subsequently noticed to be larger than the dates it is essential to know that a previous pelvic examination has been performed to exclude any other pelvic mass. In early pregnancy the commonest reason for the uterus being larger than the dates would suggest is the very simple fact that it is sitting higher in the abdomen than it normally does.

Foetal movements are felt in a twin pregnancy at the same time as they would be felt in a single pregnancy. As pregnancy advances, however, excessive movements may lead the mother to consider the possibility of twin pregnancy.

Two foetal hearts. It used to be very difficult to confirm the presence of a twin pregnancy by listening to the foetal hearts through the ordinary foetal stethoscope, but electronic aids will detect the presence of two foetal hearts as early as the 14th week.

Acute hydramnios usually accompanies a uniovular twin pregnancy. During the 24th week there is a rapid and dramatic enlargement in the size of the uterus. This may be so sudden that it causes considerable pain and discomfort, and the abdominal girth may increase from about 70 cm. to more than 112 cm. within a few days. Acute hydramnios only occurs in uniovular (identical) twin pregnancies because the circulation of one twin becomes so dominant that the second twin is forced to produce a large quantity of amniotic fluid to prevent itself from getting heart failure. In binovular twins there is a separate circulation serving each twin so that this does not occur.

Many foetal parts. The midwife’s or doctor’s suspicion is aroused if an excessive number of foetal parts are felt within the abdomen, especially if two heads are palpated.

Small baby. If a baby’s head is easily palpable and is rather small for the size of the uterus or for the duration of the pregnancy, then the presence of a twin pregnancy is always considered.

Pre-eclampsia does not usually occur until after the 32nd week of pregnancy, but a raised blood pressure or excessive weight gain before the 28th week is associated with twin pregnancy.

Two babies. The presence of a twin pregnancy is established if the midwife or doctor is absolutely certain that two babies are palpable within the uterus, or if a foetal heart detecting machine can positively identify two foetal hearts.

Ultrasound. The routine use of ultrasound scan in early pregnancy is leading many units to diagnose twins in the first 20 weeks. The test is also a useful confirmatory test where suspicion is raised.

X-ray. The X-ray may be justified if ultrasound is not available.

Duration of Pregnancy

The average duration of a twin pregnancy is 36 to 38 weeks, but the incidence of prematurity in twin pregnancy is becoming less.

Abortion in early pregnancy does not appear to be more common in twin pregnancy than single pregnancy. Hydramnios, anaemia and pre-eclampsia predispose to premature labour of which the incidence is fairly high at about the 32nd week of pregnancy. If a twin pregnancy can safely negotiate these hazards, then there is no reason why it should not proceed to 38 weeks or to term.


There are many complications which may occur in a twin pregnancy, some being minimal and of no real significance, but others are serious.

Nausea. There may be an increase in the amount of nausea during the first 3 months of pregnancy and this can continue beyond the end of the 14th week, when it is normally expected to stop.

Vomiting. Similarly, vomiting may be more severe in twin pregnancy and may continue beyond the 14th week.

Anaemia is particularly liable to occur in a twin pregnancy, especially iron deficiency and megaloblastic anaemia due to folic acid deficiency, since there are the demands of two babies instead of one.

Acute hydramnios may occur at the 24th week of pregnancy; this is indicative of a uniovular, or identical, twin pregnancy. The uterus may enlarge quite suddenly and dramatically, sufficient to cause considerable discomfort and even abdominal pain.

Pre-eclampsia is more liable to develop in twin pregnancies than in single pregnancy, but what is more important is that it is liable to develop at an earlier stage of pregnancy.

Oedema may occur partly due to pre-eclampsia but also because the presence of the large uterus causes some obstruction to the return of blood from the legs.

Excessive weight gain during a twin pregnancy is associated with preeclampsia, oedema and fluid retention.

Premature labour is more likely-in a twin pregnancy than in a single one, partly because of the greater likelihood of pre-eclampsia and partly because of the overdistension of the uterus.

Abdominal discomfort and shortness of breath may be caused by the overdistended uterus as it pushes up against the diaphragm, especially when a woman is sitting’down so that her uterus is compressed against her chest.

Piles and varicose veins are more frequent because of the increased level of circulating progesterone which causes dilatation of the blood vessels, and also because of the obstruction in circulation resulting from the overdistended uterus.

Heartburn and indigestion may also become frequent because of the pressure of the distended uterus on the stomach.

Very rarely one twin dies at an early stage of the pregnancy but the amount of hormone produced by the other is sufficient to prevent abortion and for the pregnancy to continue to term when one normal baby and one dead baby are delivered.

Care of Twin Pregnancy

A twin pregnancy requires special care for several reasons:

1 To avoid anaemia.
2 To avoid pre-eclampsia.
3 To prevent premature labour.
4 To alleviate many of the minor symptoms which can be annoying.

Anaemia can be prevented by making absolutely certain that good, satisfactory meals are eaten, and that iron and vitamin tablets are taken as instructed together with an extra supply of folic acid. The blood is tested more frequently than if only one baby is expected.

Pre-eclampsia can be prevented by rigid control of weight gain and by reducing salt intake. Throughout the whole of pregnancy weight gain should be no more than 13 kg. and any swelling of the feet and ankles should be discussed with the doctor. Plenty of rest is also important.

Premature labour is avoided if the mother has no anaemia or preeclampsia and has rested sufficiently. Many obstetricians are recom-mending hospital rest for 3 or 4 weeks to help prevent prematurity. Many of the minor symptoms will not occur if care is taken to rest as much as possible and not to become anaemic, or to gain too much weight. Elastic supporting stockings may be neccssary; some special cream or suppositories if haemorrhoids are troublesome, and an extra supply of alkali for severe indigestion or heartburn.


The most serious complication of twin labour is prematurity. If premature labour begins, intravenous salbutamol or other drugs may be given to prevent it continuing. Labour may be longer because the muscle fibres in the overdistended uterus are not as efficient as usual. This does not mean that the labour is harder or more difficult, but only that the uterine contractions are not so powerful.

Abnormal positions of one or both of die babies may give rise to complications. The commonest position for the babies is for each baby to arrive head first. However, breech presentation does occur in about 40 per cent of all the babies delivered as twins. Postpartum haemorrhage is more likely following a twin pregnancy because the overdistended uterus has difficulty in contracting and because the area of the placental site is twice as large as normal.

Twin labour must always be in hospital.

If premature labour persists, analgesic drugs may be given though epidural anaesthesia is preferable so that respiratory depressant drugs do not get to the premature baby. The small babies arc usually given during the first stage. Particular care is taken in the delivery of the premature twins so that they are not injured. This may mean that the babies are delivered by forceps for their own protection, and an episiotomy is always performed. Resuscitation equipment and incubators are ready to receivc the babies as soon as they are delivered. Twin babies who are premature have exactly the same chance of progressing normally as do single babies of the same maturity and weight. Twin babies, however, are very often smaller and so the risk is slightly greater.

If labour is at term the management of the first stage is the same as for a single pregnancy, except that two foetal hearts arc listened to instead of one. The mother may require more sedation because labour may be slightly prolonged. The second stage of labour is conducted absolutely normally if the first baby is presenting head first as it usually docs. He is delivered in the usual manner, the cord is clamped and divided and the uterus is immediately palpated to ascertain the position of the second baby. If the second twin is presenting head first then no action need be taken. Uterine contractions recommence after a few minutes and the head descends into the pelvis, the membranes are artificially ruptured and the baby is delivered quite naturally and normally. During delivery of the second baby an injection of ergometrine or syntometrine is given to ensure that the uterus contracts properly.

The third stage of labour is exactly the same as for a single pregnancy.

If the second baby is presenting as a breech, it is delivered as a breech baby. When the second baby is lying across the abdomen with neither the breech nor the head ready to engage in the pelvis, external cephalic version is usually attempted. The baby is gently rotated so that his head presents over the brim of the pelvis and the baby can then be delivered, head first, in the normal manner. Occasionally, however, if uterine contractions start almost immediately after the first baby has been delivered, it may be impossible to turn the second twin, and an internal version is performed. The woman is given a general anaesthetic and after appropriate cleaning and draping the membranes are artificially ruptured. One of the baby’s feet is gently grasped between two fingers and the foot is gendy pulled down through the cervix. This turns the baby into a breech presentation and gentle pulling on the foot results in delivery of the leg, then of the buttocks, following which the rest of the baby is delivered as a breech presentation. The third stage is normal.

Weight and Size of Babies

Uniovular, or identical, twins arc seldom the same size because even in the uterus one twin becomes dominant and grows more quickly than his brother or sister. After delivery, however, the smaller twin soon makes up the lost ground and two or three months later they will both be the same weight. Binovular, or non-identical, twins are naturally of different sizes and the larger twin nearly always remains larger. The weight of the twin depends on many factors and if premature they will naturally be small. The average weight of twins delivered at term is approximately 2-5 kg. which is perfectly normal.

The Puerperium

A twin puerperium is not liable to complications that are any different from the puerperium after a single pregnancy, always accepting that a woman who has become anaemic or had a postpartum haemorrhage or a prolonged labour, which are more common in a twin pregnancy, is liable to develop anaemia or infection during the puerperium.

Care of twin babies does not differ in any way from that of a single baby. If the babies are premature their management is exactly the same as for any single baby weighing less than 2-5 kg. The decision as to whether the babies are to be breast-fed or bottle-fed is very much an individual problem and should be discussed with the midwife or doctor. As two babies have to be fed instead of one increased attention must be given to the feeding schedule and as much rest as possible taken.