The incidence of twin pregnancy is approximately 1 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 are 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 fact. that the more babies she has the more likely are 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 winning in either family there is a possibility of having a twin pregnancy. It tends to skip generations; it is quite common for grandmother and granddaughter 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 labor and also because of an increased incidence of other complications of pregnancy.
Types of twinning
Twins may be identical or non-identical.
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 color 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 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, non-identical 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 biovular (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 binocular 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.
Modern 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 fetal hearts. Before ultrasound, about 5 percent of twin pregnancies reached 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 multiple pregnancies, 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 tumors 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 is larger than the dates would suggest is the very simple fact that it is sitting higher in the abdomen than it normally does.
Fetal 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 fetal hearts. It used to be very difficult to confirm the presence of a twin pregnancy by listening to the fetal hearts through the ordinary fetal stethoscope, but electronic aids will detect the presence of two fetal 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 fetal parts. The midwife’s or doctor’s suspicion is aroused if an excessive number of fetal 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 the 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 fetal-heart-detecting machine can positively identify two fetal hearts.
Ultrasound. The routine use of ultrasound scan in early pregnancy ensures that the diagnosis is made before any of the above physical signs arouse suspicion. Twins can be diagnosed by scan as early as the end of the 6th week of pregnancy and the examination can always be used to confirm any suspicions.
X-rays. The X-ray is not justified because ultrasound ought to be available.
Duration of pregnancy
The average duration of a twin pregnancy is 36 to 38 weeks, but the incidence of prematurity in the twin pregnancy is becoming less.
Abortion in early pregnancy does not appear to be more common in twin pregnancy than a single pregnancy. Hydramnios, anemia, and pre-eclampsia predispose to the premature labor 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.
Anemia is particularly liable to occur in a twin pregnancy, especially iron deficiency and megaloblastic anemia 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.
Edema 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 pre-eclampsia, edema and fluid retention.
Premature labor 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 over-distension of the uterus.
Abdominal discomfort and shortness of breath may be caused by the over-distended 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 over-distended 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:
- To avoid anemia.
- To avoid pre-eclampsia.
- To prevent premature labor.
- To alleviate many of the minor symptoms which can be annoying.
Anemia 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 labor is usually avoided if the mother has no anemia or pre-eclampsia and has rested sufficiently. Many obstetricians are recommending 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 anemic, or to gain too much weight. Elastic supporting stockings may be necessary; some special cream or suppositories if hemorrhoids are troublesome, and an extra supply of alkali for severe indigestion or heartburn.
Twin labor must always be in a hospital.
Premature labor is that occurring before the 37th week of pregnancy. It is common with twin pregnancies and if it occurs before 35 weeks, is a potentially serious complication. If premature labor begins, drugs such as intravenous salbutamol or Ritodrine may be given to try and prevent it continuing. If premature labor cannot be stopped then delivery will occur. Depending on the duration of the pregnancy and the position of the babies this may be allowed to proceed to vaginal delivery or Caesarean section may be indicated. Each case will be taken individually and the method of delivery decided upon will be that which is safest for both mother and babies. If labor proceeds, an epidural anesthetic is preferable to pain relieving injections that may depress the newborn babies’ respiration, and if a Caesarean section is performed this may either be with an epidural or under general anesthetic depending on the circumstances.
For a premature twin delivery, resuscitation equipment and incubators will be ready to receive the babies and pediatricians and an anesthetist will be in attendance as well as the obstetrician and midwife. Premature twins must be delivered gently to avoid injury, and if delivered vaginally then this may mean using forceps and an episiotomy to ensure the safest, gentlest delivery possible.
Premature twins may well need to spend some time in the Special Care Baby Unit, but they have the same chance of progressing normally as do single babies of the same maturity and weight. They are usually slightly smaller than single babies of the same gestation and this may make the risk of problems slightly greater.
Labor at term
The term is usually understood as 40 weeks gestation, but it is uncommon for a twin pregnancy to get to or past 40 weeks. Labour between 36 and 40 weeks may be considered ‘at the term’ for twins and, indeed, some obstetricians prefer to see twins delivered by the end of the thirty-eighth week. Twin labor may be longer than with a single baby because the muscle fibers in the overdistended uterus are not as efficient as usual. This does not mean that the labor is harder or more difficult, but only that the uterine contractions are not so powerful. Abnormal positions of one or both of the babies may give rise to problems although the commonest position for the babies is for each to arrive head first. However, the breech presentation does occur in about 40 percent of all babies delivered as twins.
The first stage of labor is managed the same as for a single pregnancy except that two fetal hearts are listened to instead of one. Many people prefer an epidural for pain relief for twin labors in case assistance is required with delivering one or other, or both babies. The second stage of labor is conducted normally if the first baby is presenting head first as it usually does. He is delivered in the usual manner and the cord clamped and divided. Immediately after the delivery of the first twin, the uterus is palpated to ascertain the position of the second baby. If the second twin is presenting head first, no action need be taken and the uterus will recommence contracting after a few minutes. Once the head descends into the pelvis, the membranes of the second twin are ruptured and the is delivered naturally and normally. If the second twin is presenting as a breech and the pelvis is suitable, then the baby is delivered as a breech baby. If the second baby is lying across the abdomen with neither the head or breech presenting, external cephalic version is attempted. The baby is gently turned so that its head presents over the pelvis and the baby can then be delivered normally. If it is impossible to turn the second twin, then either internal version or Caesarean section is performed. Internal version is performed after artificially rupturing the membranes. One of the baby’s feet is gently grasped by the obstetrician and the foot is gently pulled down through the cervix. This turns the baby into a breech presentation and delivery is effected by gentle pulling of the foot—a breech delivery is performed, usually with forceps to deliver the after-coming head carefully. Internal version and breech extraction or Caesarean section will need either epidural or general anesthesia.
As the second baby is delivered, an injection of Ergometrine or syntonins is given to the mother. This is usually given intravenously into the drip and it ensures the uterus contracts properly. The placenta or placental are then delivered. Postpartum hemorrhage is more common following a twin delivery because the overdistended uterus has difficulty in contracting and because the placental site is twice as large as normal. The third stage of labor is the same as for a single pregnancy.
With careful preparation for labor and with correct management, the potential complications of twin labors are minimized.
Weight and size of babies
Uniovular or identical, twins usually reach the same weight two or three months after delivery, despite differences at birth. Binocular or non-identical, twins are naturally of different sizes and the larger twin at birth usually remains larger. The average normal weight of twins delivered at term is approximately 2.5 kg.