Twins could either be non-identical or identical. Non-identical twins arise from two ova fertilised by two separate sperms. Each twin has a separate placenta. They have separate sacs, each with an inner membrane (amnion) and outer membrane (chorion). Two out of three of all twin pregnancies are non-identical (dichorionic diamniotic).
Identical twins occur in one out of three twin pregnancies. They arise from a single ovum fertilised by a single sperm, which then divides into two identical embryos. If the division occurs in the first three days after fertilisation, the twins will have their own placenta and membranes (dichorionic diamniotic). Their ultrasound appearance will be the same as non-identical twins.
If the division occurs between the fourth and ninth day, the twin will share the same placenta and chorion, but have separate amnions (monochorionic diamniotic). If the division occurs after the ninth day, the twins will be in a single sac (monochorionic monoamniotic).
Two in three identical twins are monochorionic diamniotic; one in three dichorionic diamniotic and one in 100 monochorionic monoamniotic.
The diagnosis of twin pregnancy is enhanced by routine ultrasound. Without it, about four in 10 twin pregnancies will not be diagnosed until 26 weeks gestation, and about two in 10 remain undiagnosed until term.
Ultrasound in the first or second trimester will usually determine with more than 95 per cent accuracy if the twins share the same placenta. The detection of foetal anomalies, of which the incidence is three times more in twin pregnancy, is best done with ultrasound between 16 and 20 weeks.
Foetal growth can be reliably assessed with serial ultrasounds in the second and third trimesters.
The incidence of twin pregnancies has increased worldwide and is the major reason for the increase in pre-term births. More than five in 10 twin, and more than 98 in 100 triplet, pregnancies deliver before 36 weeks gestation. Because of the increased incidence of pre-term births, these babies will need to spend some time in the neonatal intensive care unit.
There is no single method that predicts the likelihood of pre-term labour and birth. However, there is evidence that pre-term labour and birth can be predicted by vaginal examination, which detects premature change in the state of the mother's cervix.
There is also evidence that ultrasound measurement of the cervical length and/or the presence of cervico-vaginal foetal fibronectin is predictive of pre-term labour.
It is essential that there be a discussion with the obstetrician about the modes of delivery early in pregnancy. They are vaginal delivery or Caesarean section, which can be planned or unplanned. Because the likelihood of complications with twin deliveries is increased, an early decision will also have to be made about the place of delivery as neonatal intensive care units (NICU) are only found in certain hospitals.
Labour in twin pregnancy is the same as that of a single pregnancy. The lie and presentation of each foetus is checked on admission, preferably with ultrasound. An intravenous line would be inserted, and blood sent for screening and/or matching.
The obstetrician, anaesthetist, paediatrician, neonatal intensive care unit and operating theatre are informed early that there is a twin pregnancy admitted.
Both foetuses are monitored closely with continuous electronic monitoring of their heart rates. A foetal scalp electrode may be applied to the first twin when the membranes rupture. Labour may need to be augmented.
Pain relief with an epidural is often recommended as it facilitates assisted delivery, should problems arise.
The indications for any intervention for either twin are evidence-based.
About five in 10 twins are delivered vaginally. The delivery process is the same as that of a single pregnancy.
If the first twin is presenting by the head and there are no obstetric or medical problems, the obstetrician will usually recommend a vaginal delivery. The occasions when vaginal delivery is assisted with a vacuum extraction (ventouse) or forceps are similar to that of a single pregnancy.
After the first twin has been delivered, the obstetrician will perform an abdominal and vaginal examination to determine the longitudinal axis (lie) and presenting part of the second twin.
If the lie is longitudinal, the membranes of the second twin will be ruptured artificially (amniotomy), and labour augmented with an intravenous drip containing oxytocin, if the contractions have slowed down or stopped after the first twin's delivery.
If the lie is not longitudinal, an external cephalic version (ECV) may be carried out, followed by amniotomy and augmentation of labour.
Alternatively, the obstetrician may insert a hand into the birth canal to grasp one or both foetal feet and draw it through the cervix (internal podalic version), followed by a breech extraction. Internal podalic version (IPV) requires a skilled obstetrician and is not done often nowadays.
If the second twin, with an estimated weight between 1.5 to 4.0kg, is presenting by breech, a vaginal delivery can be carried out, provided the obstetrician is comfortable with, and skilled in, vaginal breech delivery.
The maternal and neonatal outcomes of breech extraction with or without IPV are the same as ECV in twins weighing more than 1.5kg.
The optimal delivery interval between the first and second twin has been debated often. It is reasonable to expedite delivery of the second twin by amniotomy, intravenous oxytocin and assisted vaginal delivery.
Alternatively, it is also reasonable to allow a longer interval between the deliveries, provided the foetal heart rate, monitored electronically, is reassuring. However, if a breech extraction with or without IPV is considered, it should be done without delay.
The reasons for planned Caesarean section include a breech (buttocks, feet, or knees) presentation of the first twin, a transverse lie of the first twin, twins with a shared placenta, conjoined twins, triplets and other higher order pregnancies, and indications as in single pregnancies, eg placenta sited over the birth canal (praevia), maternal hypertension, and difficulty in previous delivery.
Sometimes, a mother may choose to have a Caesarean section even when there are no complications.
An unplanned Caesarean section will be carried out should any problems arise during labour or after the delivery of the first twin. The former includes maternal hypertension, non-reassuring foetal heart rate(s), an umbilical cord dropping into the birth canal below the foetal presenting part (cord prolapse), poor progress, or failed assisted vaginal delivery. The latter occurs in less than five in 100 twin deliveries, and is usually because of non-reassuring foetal heart rate.
The longer the foetuses are in the mother's uterus, the higher are their chances of being healthy. Pre-term birth has immediate and long-term health implications for the babies. The earlier the birth, the higher is the risk to health.
One out every two babies born before 24 weeks live, and the other may die or have long-term problems. On the other hand, the survival rate of babies born after 32 weeks is high, and most do not develop long-term complications.
Pre-term babies have immediate problems with breathing, feeding and maintaining temperature. This requires nursing in an incubator, oxygen by mask or ventilator, and feeding by a tube inserted into the stomach or into a blood vessel.
Pre-term babies born in a hospital with a NICU have the best outcomes. However, not every hospital has a NICU. As such, it may be necessary to transfer the mother and babies to another hospital with a NICU, preferably before delivery, or if not possible, immediately after the babies' births.
The longer term problems of pre-term babies include developmental delay, asthma, behavioural problems and learning difficulties. The earlier pre-term birth occurs, the more likely the babies will be readmitted to hospital in the first few months of life, compared to those born at full-term.
There are several factors that contribute to the successful management of a twin pregnancy and delivery. The most important are a competent and dedicated obstetrician, paediatrician, anaesthetist and nursing staff with adequate medical equipment.
In short, effective teamwork makes the difference.