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Extracted from Survival Guide to Midwifery, 2nd Edition (2012) Diane M. Fraser and Margaret A. Cooper, Oxford; Churchill Livingstone: 2012. Courtesy Elsevier
The term multiple pregnancy describes the development of more than one fetus in the uterus at the same time.
Types of twin pregnancy
Twins will be either monozygotic (MZ) or dizygotic (DZ). In the UK, approximately two-thirds will be DZ and one-third MZ. Superfecundation is the term used when twins are conceived from sperm from different men if a woman has had more than one partner during a menstrual cycle. It is not known how often this happens, but if suspected then paternity can be checked by DNA testing. Superfetation is the term used for twins conceived as the result of two coital acts in different menstrual cycles. This is thought to be very rare.
Monozygosity and dizygosity
Monozygotic or uniovular twins
Dizygotic or binovular twins
Diagnosis of twin pregnancy
This is usually through ultrasound examination. Diagnosis can be made as early as 6 weeks into the pregnancy, or later at the routine detailed structural scan between the 18th and 20th weeks. A family history of twins should alert the midwife to the possibility of a multiple pregnancy. Occasionally (1 in 12 000 live births), one fetus may die in the second trimester and become a fetus papyraceous, which becomes embedded in the surface of the placenta and expelled with the placenta at delivery.
A multiple pregnancy tends to be shorter than a single pregnancy. The average gestation for twins is 37 weeks, for triplets 34 weeks, and for quadruplets 33 weeks.
Effects of pregnancy
Exacerbation of common disorders
More than one fetus and the higher levels of circulating hormones often exacerbate the common disorders of pregnancy. Sickness, nausea and heartburn may be more persistent and more troublesome than in a singleton pregnancy.
Iron deficiency and folic acid deficiency anaemias are common. Early growth and development of the uterus and its contents make greater demands on the maternal iron stores; in later pregnancy (after the 28th week) fetal demands may lead to anaemia. Routine prescription of iron and folic acid supplements is not necessary.
This is particularly associated with monochorionic twins and with fetal abnormalities. Polyhydramnios will add to any discomfort that the woman is already experiencing. Acute polyhydramnios can lead to miscarriage or premature labour.
Impaired venous return from the lower limbs increases the tendency to varicose veins and oedema of the legs. Backache is common and the increased uterine size may also lead to marked dyspnoea and to indigestion.
There can be an increase in complications of pregnancy.
Early diagnosis of a twin pregnancy and of chorionicity is extremely important in order to support and advise the parents.
Preparation for breastfeeding
Mothers should be advised not only that it is possible to breastfeed two, and in some cases three, babies, but also that, nutritionally, this is the best way for her to feed her babies.
Labour and birth
Onset of labour
The higher the number of fetuses the mother is carrying, the earlier the labour is likely to start. Term for twins is usually considered to be 37 weeks rather than 40, and approximately 30 per cent of twins are born preterm. In addition to being preterm the babies may be small for gestational age and therefore prone to the associated complications of both conditions. If spontaneous labour begins very early the mother may be given drugs to inhibit uterine activity. Known causes of preterm labour, for example urinary tract infection, should be treated with antibiotics. It is very unusual for a twin pregnancy to last more than 40 weeks; many obstetricians advise induction of labour at 38 weeks. If the first twin is in a cephalic presentation, labour is usually allowed to continue normally to a vaginal birth, but if the first twin is presenting in any other way, an elective caesarean section is usually recommended.
Management of labour
Induction of labour usually occurs around 38 weeks' gestation. The presence of complications such as pregnancy-induced hypertension, intrauterine growth restriction or twin-to-twin transfusion syndrome may be reasons for earlier induction. The majority will go into labour spontaneously. There is an increased incidence of dysfunctional labour in twin pregnancies, possibly because of overdistension of the uterus.
Mobilisation or the adoption of whichever position the mother finds most comfortable should be encouraged. A foam rubber wedge under the side of the mattress will help to prevent supine hypotensive syndrome by giving a lateral tilt. A birthing chair or a reclining chair may be more comfortable than a delivery bed. Regional epidural block provides excellent analgesia and, if necessary, allows easier instrumental deliveries and also manipulation of the second twin. The use of inhalation analgesia may be helpful, either before the epidural is in situ or during the second stage if the effect of the epidural is wearing off.
Management of the births
The second stage of labour may be confirmed by a vaginal examination. The obstetrician, paediatric team and anaesthetist should be present for the births because of the risk of complications.
Complications associated with multiple pregnancy
The high perinatal mortality associated with twinning is largely due to complications of pregnancy, such as the premature onset of labour, intrauterine growth restriction and complications of delivery.
Acute polyhydramnios may occur as early as 18-20 weeks. It may be associated with fetal abnormality but it is more likely to be due to TTTS.
Twin-to-twin transfusion syndrome
Also known as fetofetal transfusion syndrome (FFTS), this can be acute or chronic. The acute form usually occurs during labour and is the result of a blood transfusion from one fetus (donor) to the other (recipient) through vascular anastomosis in a monochorionic placenta. Both fetuses may die of cardiac failure if not treated immediately. Chronic FFTS can occur in up to 35% of monochorionic twin pregnancies and accounts for 15-17% of perinatal mortality in twins. The placenta transfuses blood from one twin fetus to the other. Fetal and/or neonatal mortality is high but some infants may be saved by early diagnosis and prenatal treatment. Selective fetocide is sometimes considered.
This is particularly associated with MZ twins.
This extremely rare malformation of MZ twinning results from the incomplete division of the fertilised oocyte. Delivery has to be by caesarean section. Separation of the babies is sometimes possible and will depend on how they are joined and which internal organs are involved.
Acardiac twins (twin reversed arterial perfusion - TRAP)
One twin presents without a well-defined cardiac structure and is kept alive through placental anastomoses to the circulatory system of the viable fetus.
Parts of one fetus may be lodged within another fetus; this can happen only in MZ twins.
The fetuses can restrict each other's movements, which may result in malpresentations, particularly of the second twin. After the birth of the first twin, the presentation of the second twin may change.
Premature rupture of the membranes
Malpresentations due to polyhydramnios may predispose to preterm rupture of the membranes.
Prolapse of the cord
This is associated with malpresentations and polyhydramnios and is more likely if there is a poorly fitting presenting part. The second twin is particularly at risk.
Malpresentations are a poor stimulus to good uterine action and a distended uterus is likely to lead to poor uterine activity and consequently prolonged labour.
Monoamniotic twins risk cord entanglement with occlusion of the blood supply to one or both fetuses. Delivery is usually at around 32-34 weeks and by caesarean section.
This is a rare but serious complication. There are two types: one occurs when the first twin presents by the breech and the second by the vertex, the other when both are vertex presentations. In both instances the head of the second twin prevents the continued descent of the first.
Delay in the birth of the second twin
After delivery of the first twin, uterine activity should recommence within 5 minutes. Birth of the second twin is usually completed within 45 minutes of the first birth. In the past the birth interval was limited to 30 minutes in an attempt to minimise complications. With the introduction of fetal heart rate monitoring the interval time between babies is not so crucial as long as the fetal condition is monitored. Poor uterine action as a result of malpresentation may be the cause of delay. The risks of delay are:
After the birth of the first twin the lower uterine segment begins to reform and the cervical canal may have to dilate fully again. The midwife may need to 'rub up' a contraction and to put the first twin to the mother's breast to stimulate uterine activity.
Premature expulsion of the placenta
The placenta may be expelled before delivery of the second twin.
Poor uterine tone as a result of overdistension or hypotonic activity is likely to lead to postpartum haemorrhage.
The possibility of an unexpected undiagnosed second baby should be considered if the uterus appears larger than expected after the birth of the first baby or if the baby is surprisingly smaller than expected. If an uterotonic drug has been given after the birth of the anterior shoulder of the first baby, the second baby is in great danger and delivery should be expedited. He or she will require active resuscitation because of severe asphyxia.
Care of the babies
Immediate care at delivery is the same as for a single baby. Identification of the infants should be clear and the parents should be given the opportunity to check the identity bracelets and cuddle their babies.
Both babies may be breastfed, either simultaneously or separately. The mother may choose to feed artificially.
The more stimulation the breasts are given, the more plentiful is the milk supply.
Care of the mother
Involution of the uterus will be slower because of its increased bulk. 'Afterpains' may be troublesome and analgesia should be offered. A good diet is essential, and if the mother is breastfeeding, she requires a high-protein, high-calorie diet. Once the mother is at home she must be encouraged to rest and eat a well-balanced diet. The incidence of postnatal depression has been shown to be significantly higher in twin mothers.
Triplets and higher-order births
A woman expecting three or more babies is at risk of all the same complications as one expecting twins, but more so. She is more likely to have a period in hospital resting before the babies are born and they will almost certainly be delivered prematurely. Perinatal mortality rates are higher for triplets than twins and the incidence of cerebral palsy is also increased. The mode of delivery for triplets or more babies is usually by caesarean section. It is essential that the paediatric team be present. The special dangers associated with these births are:
Perinatal mortality and long-term morbidity are both more common among multiple births than singletons. The perinatal mortality rate for twins is about four times that of singletons, and that of triplets 12 times higher.
This is the reduction of an apparently healthy higher-order multiple pregnancy down to two or even one embryo so the chances of survival are much higher. The procedure may be offered to parents who have conceived triplets or more. The procedure is usually carried out between the 10th and 12th weeks of the pregnancy. Various techniques may be used, involving the insertion of a needle under ultrasound guidance either via the vagina or, more commonly, through the abdominal wall into the fetal thorax. Potassium chloride is usually used, although some doctors prefer saline. All embryos remain in the uterus until birth.
This may be offered to parents with a multiple pregnancy when one of the babies has a serious abnormality. The affected fetus is injected as described in embryo reduction, so allowing the healthy fetus to grow and develop normally.
NICE. (2011). Multiple pregnancy: antenatal care for twin and triplet pregnancies. Clinical guideline 129. https://www.nice.org.uk/guidance/CG129
Royal College of Obstetricians & Gynaecologists. Management of monochorionic twin pregnancy. Green-top Guideline No 51. https://www.rcog.org.uk/globalassets/documents/guidelines/t51managementmonochorionictwinpregnancy2008a.pdf