Epidemiology

Normal incidence of twins is 1 in 90 pregnancies (approximately 1/3 are monozygotic) and of triplets 1 in 8,100 pregnancies.

However, use of in vitro fertilisation (IVF) and ovulation induction techniques have greatly increased the incidence of multiple pregnancies.^[1] Figures from the North of England twin register suggest a twinning rate of 13.6-16.6/1,000 maternities (or 1 in 60-74 pregnancies).^[2]

Risk factors

For dizygotic twinning include:

• Previous multiple pregnancy.
• Family history (maternal side).
• Increasing maternal age.
• Racial origin (more common in women of West African ancestry, less common in those of Japanese ancestry).
• Assisted conception.

Presentation

• Nearly all multiple pregnancies are now diagnosed in the first trimester by ultrasound. However, some twins die and are absorbed in the first half of pregnancy ('the disappearing twin' syndrome) and early scanning increases awareness of this phenomenon.
• Early symptoms may include hyperemesis and other exaggerated pregnancy-related symptoms. The uterus may be palpated abdominally earlier than 12 weeks of gestation.
• In the second half of pregnancy, may present with large-for-dates uterine size, higher than expected weight gain, >2 fetal poles on palpation and 2 or more fetal heart rates heard on auscultation.

Prenatal diagnosis^[3]

The women should be informed about the greater likelihood of Down's syndrome in twin and triplet pregnancies before screening. They should be made aware of the different options for screening and the higher false-positive rate of screening tests in twin and triplet pregnancies. As a result of this they have a greater likelihood of being offered invasive testing and of complications occurring from this testing. Screening should be performed between approximately 11 weeks 0 days and 13 weeks 6 days:^[4]

• The fetal positions should be noted.
• The risk per pregnancy in monochorionic pregnancies and for each baby in dichorionic and trichorionic pregnancies should be calculated.
• Women whose risk of Down's syndrome exceeds 1:150 should be offered a referral to a fetal medicine specialist in a tertiary level fetal medicine centre.
• Twin pregnancies should use the 'combined test' and consider second trimester serum screening if the woman books too late for first trimester screening. Explain the potential problems (particularly the increased likelihood of pregnancy loss associated with double invasive testing because the risk cannot be calculated separately for each baby).
• Triplet pregnancies should use nuchal translucency and maternal age. They should not use second trimester serum screening.

If one fetus is detected as abnormal, selective termination (if desired) must be accurately targeted. Selective termination in monochorionic pregnancies risks co-twin sequelae.^[5]

Antenatal care^[4]

Referral

Twin pregnancies should be referred to obstetricians for shared care, due to the higher risk they present. There should be a special care baby unit (SCBU) available. A tertiary level fetal medicine centre referral is indicated for:

• Monochorionic monoamniotic twin pregnancies.
• Monochorionic monoamniotic triplet pregnancies.
• Monochorionic diamniotic triplet pregnancies.
• Dichorionic diamniotic triplet pregnancies.
• Pregnancies complicated by any of the following:
  • Asymmetrical fetal growth.
  • Fetal anomaly.
  • Death of one fetus.
  • Twin-twin transfusion syndrome.

  Scanning

  Women with multiple pregnancies should be offered a first trimester ultrasound scan when approximately 11 weeks 0 days to 13 weeks 6 days:^[4]
  • This is to estimate gestational age, determine chorionicity and screen for Down's syndrome.
  • Ideally these should all be performed at the same scan.
  • Chorionicity should be determined using the number of placental masses, the lambda or T-sign and membrane thickness.
• Monochorionic twins should be scanned fortnightly to detect twin-twin transfusion from 16 to 24 weeks of gestation.

Multiple pregnancies should be monitored carefully for intrauterine growth restriction (IUGR):

• Differences in fetal weight should be monitored using two or more parameters at each ultrasound scan from 20 weeks.
• Ideally scans should be at intervals of less than 28 days.
• If there is a 25% or greater difference in size between twins or triplets, this is a clinically important indicator of IUGR. A referral to a tertiary level fetal medicine centre should be offered.

First-line management is usually laser surgery of inter-twin vascular placental anastomoses where the syndrome develops before 26 weeks of gestation.^[6] Other options include intrauterine blood transfusions, serial amnioreduction or elective delivery.^[7]

All twin pregnancies (regardless of chorionicity) are regularly scanned after about 30 weeks to monitor growth and fetal well-being, with early delivery induced in cases of growth cessation and/or poor Doppler blood flow indices.

Maternal health

There is a higher incidence of anaemia in women with twin and triplet pregnancies. FBC should be taken at 20-24 weeks to identify a need for early supplementation with iron or folic acid. This should be repeated at 28 weeks as in routine antenatal care.

Maternal complications (eg pre-eclampsia) are more common and carers should be vigilant for early signs.^[8] Women are advised to take 75 mg of aspirin daily from 12 weeks until the birth of the babies if they have one or more of the following risk factors for hypertension:

• First pregnancy.
• Age 40 years or older.
• Pregnancy interval of more than 10 years.
• BMI of 35 kg/m² or more at first visit.
• Family history of pre-eclampsia.

Because of the extra load on the heart, previously women were recommended bed rest. However, there is no clear evidence to support this.^[9]

Timing of delivery^[4]

• Women with uncomplicated monochorionic twin pregnancies should be offered elective birth from 36 weeks 0 days, after a course of antenatal corticosteroids has been advised.
• Women with dichorionic twin pregnancies should be offered elective birth from 37 weeks 0 days.
• Women with triplet pregnancies should be offered elective birth from 35 weeks 0 days, after a course of antenatal corticosteroids has been advised.

If the woman declines elective birth, weekly appointments should be offered with the specialist obstetrician. An ultrasound scan and biophysical profile assessment should be part of each appointment, with fortnightly fetal growth scans.

Intrapartum management

On admission in labour:

• Obtain intravenous (IV) access.
• Monitor fetal heart rates separately.
• Check the position of the lead fetus:
  • In 45% of cases, both fetuses present as cephalic.
  • In 25% of cases, the first twin is cephalic, the second breech.
  • In 10% of cases, breech/breech or first twin breech/second cephalic.

Vaginal delivery of twins

With no complicating factors, the mother can go into spontaneous labour provided the first twin has a cephalic presentation.

• Where the first twin presents in a breech position or transversely, Caesarean section is preferred.
• In most cases, vaginal birth proceeds as normal.
• With rupture of membranes, check for prolapse of umbilical cord.
• Immediately after the first baby is born:
  • Determine the position of the second fetus by vaginal examination.
  • If longitudinal, rupture the second amniotic sac (once the presenting part is engaged, usually after a couple of contractions) and proceed to delivery.
  • If transverse, external cephalic or internal podalic version may be attempted to bring into longitudinal position.
  • If successful, as confirmed by vaginal examination, then rupture the second amniotic sac when the fetal head is engaged.
  • If unsuccessful, deliver by Caesarean section.
• Contractions can reduce after the birth of the first fetus and, if they do not quickly return (within 15 minutes), set up an IV oxytocin infusion, following which birth of the second fetus should be straightforward. The second twin should deliver within 20-45 minutes of the first twin.
• Where there are difficulties with the delivery of the second twin or if it develops a bradycardia, a vacuum extraction (in cephalic position) or breech extraction can be performed without necessarily resorting to Caesarean delivery.
• Third stage should be actively managed by intramuscular (IM) injection of Syntometrine® or Syntocinon® as the fetal head is being born, in order to avoid postpartum haemorrhage.
• Twin deliveries should be attended by two paediatricians, two obstetricians and an anaesthetist.

Operative delivery of twins

Vaginally delivered second twins have an increased perinatal morbidity and mortality thought to be due to intrapartum anoxia.^[10] The question of whether all women with twin pregnancies should have a Caesarean section is contentious.

National Institute for Health and Clinical Excellence (NICE) guidance recognises the increased risk for the second twin in uncomplicated twin pregnancies at term but feels that evidence as to whether section for the second twin improves outcome, remains uncertain and therefore should not be routinely offered, except as part of research.^[11]

Some studies have concluded that Caesarean delivery is beneficial for the second twin, with an NNT to prevent an adverse outcome of 33.^[12] Other studies advocate that, provided labour is actively managed in experienced centres and that complicated twin pregnancies are excluded, it continues to be a safe option.^[13]

Complications^[14]

Although a naturally occurring phenomenon, multiple pregnancies are considered high-risk because:

• Smaller babies - fetuses tend to be individually smaller than those in a singleton pregnancy because of greater demand for nutrients and slower in utero growth, ie light-for-dates. Monozygotic twins tend to be smaller than dizygotic twins.
• Increased risk of prematurity - the mean gestation for twins is 37 weeks and for triplets 31 weeks. In particular, there is a higher risk of spontaneous preterm birth if they have had a spontaneous preterm birth in a previous singleton pregnancy. Neither cervical length, nor fetal fibronectin alone, should be routinely used to predict preterm birth. There is insufficient evidence to support the use of vaginal progesterone, bed rest at home or in hospital, cervical cerclage or oral tocolytics to prevent prematurity.^[15][4].
• Higher risk of congenital abnormality associated with multiple pregnancies (x 2-4 rate in singleton pregnancies).
• Higher rates of cerebral palsy found in twins (1-1.5%) and triplets (7-8%).
• Perinatal mortality rate for twins is significantly higher than singletons (x 5) and even higher for triplets (x 6). Rates are higher for monochorionic twins than dichorionic twins (49 versus 11.5/1,000).^[2]
• Higher rate of maternal pregnancy-related complications, such as hyperemesis gravidarum, polyhydramnios, pre-eclampsia, anaemia, antepartum haemorrhage.
• Higher rate of complications in labour - malpresentation, vasa praevia, cord prolapse, premature separation of placenta, cord entanglement, postpartum haemorrhage.

Complications of multiple pregnancy do not end with birth. Language and speech delay, more general cognitive delay or motor problems, behavioural problems and difficulty in parent-child interactions all appear to be more common in multiple birth children.^[16]

Also, the non-medical financial, social and emotional consequences of caring for twins or higher-order multiples need to be considered.

Prevention

Clearly, the outcomes of multiple pregnancies (particularly higher-order multiples) are poorer than singleton pregnancies. A few have challenged this consensus, based on the argument that, if more than once child is desired via fertility treatment, the risk and costs are diminished where analysis is based on born children rather than pregnancies.^[17]

Primary prevention should be aimed for. Limiting the number of embryos transferred in IVF and close counselling/monitoring of those using ovulation-induction therapies. The Human Fertilisation and Embryology Authority has criteria for single embryo transfer. If the woman does not fulfil them, a maximum of two embryos can be transferred per cycle.^[18]

Secondary prevention in the form of multifetal pregnancy reduction (MFPR) appears effective but is not acceptable to all, particularly those with a past history of infertility.^[14] Evidence is not gold standard, as RCTs are ethically and practically very difficult in these situations.^[19]

MFPR is performed early in pregnancy, usually between 9 and 12 weeks with a transabdominal (TA) or transvaginal (TV) ultrasound-guided injection of potassium chloride into the selected fetus(es). Usual practice is to reduce higher-order pregnancies to a twin pregnancy, although some now argue in favour of reduction to a singleton pregnancy, as the likelihood of taking home a baby is higher than remaining with a twin pregnancy.^[20]

Risks of a triplet pregnancy compared with a triplet-reduced-to-twin pregnancy:^[14]

Risks of MFPR:

• Miscarriage of remaining fetuses (approximately the same rate as in normal twin pregnancies).
• Emotional consequences to parents.
• Infection (rare).