Multiple gestation

• Multiple gestation means more than one fetus is present in the same pregnancy: twins, triplets or higher.
• The single most important distinction is not whether the babies came from one egg or two but whether they share a placenta (chorionicity) and an amniotic sac (amnionicity). That single fact decides how closely they must be watched.
• Twins who share a placenta (monochorionic) share blood vessels between them. About 15% of monochorionic pregnancies develop twin-twin transfusion syndrome, twin anaemia-polycythaemia sequence or twin reversed arterial perfusion sequence. ^[4]
• Multiple pregnancy is the commonest reason for a pregnancy to end before 37 weeks: up to 60% of multiple pregnancies deliver preterm, versus about 10% delivering before 32 weeks in twins compared with 1% in singletons. ^[5]
• The babies should be delivered at a planned time that depends on chorionicity: 37 weeks for uncomplicated dichorionic twins, 36 weeks for uncomplicated monochorionic diamniotic twins, 35 weeks for uncomplicated trichorionic or dichorionic triplets, and 32 to 33+6 weeks for monochorionic monoamniotic twins. ^[1]
• Management is specialist, scheduled and proactive: multidisciplinary antenatal care, serial ultrasound, and prompt referral to a tertiary fetal medicine centre if complications appear. ^[1][2]

By the end of this page you should be able to:

1. Define multiple gestation and classify it by chorionicity and amnionicity, and explain why zygosity is less important.
2. Explain the placental vascular anastomoses that drive twin-twin transfusion syndrome and related monochorionic complications.
3. Recognise the clinical clues to a multiple pregnancy and to its complications.
4. Plan screening and ultrasound surveillance using the thresholds used by NICE and ISUOG.
5. Explain the prevention of preterm birth in twin and triplet pregnancy, including when progesterone or aspirin is used.
6. Outline delivery planning (timing, mode of birth and intrapartum monitoring) and compare the NICE, ACOG/SMFM and ISUOG positions.

A multiple gestation is any pregnancy with more than one fetus. Twins are two, triplets three. Most of what follows applies to twins, because that is where the evidence is strongest; triplets and higher orders raise the same problems more sharply.

There are two independent ways to describe a twin pregnancy.

Zygosity asks how many eggs started it. Dizygotic (fraternal) twins arise from two separate eggs fertilised by two sperm; they are genetically siblings and may be different sexes. Monozygotic (identical) twins arise from one fertilised egg splitting; they share the same sex and genetic material.

Chorionicity and amnionicity ask what the fetuses actually share. Chorionicity is the number of chorions (placentas) - dichorionic means two placentas, monochorionic means one shared placenta. Amnionicity is the number of amniotic sacs - diamniotic means two sacs, monoamniotic means one shared sac.

This is the distinction that matters clinically. A monozygotic twin pregnancy can be either monochorionic (most, if splitting is early) or, rarely, dichorionic (if the split happens very early). A dizygotic pregnancy is always dichorionic diamniotic. Because monochorionicity carries the risk of inter-twin blood flow, it is chorionicity - not zygosity - that drives surveillance and outcome. ^[1]

The nine combinations used in practice are summarised below. ^[1]

Twin and higher-order multiple births rose sharply over the last four decades and have since levelled off. The rate of twin births in the United States increased 76% between 1980 and 2009, from 18.9 to 33.3 per 1,000 births, and was 30.7 per 1,000 births in 2023. ^[3][4] The triplet and higher-order birth rate peaked at 193.5 per 100,000 births in 1998 and had fallen to 93.0 per 100,000 in 2018. ^[3] The two drivers are advancing maternal age at conception and wider use of assisted reproductive technology (ART), including the transfer of more than one embryo and a higher rate of monozygotic twinning after in-vitro fertilisation. ^[2][5]

Monochorionic pregnancies account for roughly one in five twin pregnancies: about 20% in US practice and about 30% in the UK. ^[2][4] They are the pregnancies that need the strictest surveillance, because the shared placenta is the source of the unique monochorionic complications described below.

The burden of prematurity is large. Preterm birth before 37 weeks occurs in up to 60% of multiple pregnancies, and the risk of very preterm birth before 32 weeks is about 10 times higher in twins than in singletons (about 10% versus 1%). ^[5] In 2019 the stillbirth rate was 7.6 per 1,000 twin births compared with 3.8 per 1,000 singleton births. ^[5] A 2019 UK figure quoted by NICE is that about 60 of every 100 twin pregnancies and about 75 of every 100 triplet pregnancies end in spontaneous birth before 37 or 35 weeks respectively. ^[1]

Regional note: RCOG quotes the UK figure of about 30% of twin pregnancies being monochorionic, whereas US and international reviews more often quote about 20%. ^[2][4] No new guideline changes the basic thresholds, so both figures are worth remembering. ^[1][5]

• Assisted reproduction. The strongest modifiable factor. Transferring more than one embryo increases dizygotic twinning; IVF itself also increases monozygotic twinning. ^[5]
• Maternal age and parity. Older mothers and women who have had previous pregnancies have more dizygotic twins. ^[5]
• Family history. Dizygotic twinning runs in families through the mother. ^[5]
• Ethnicity and population. Dizygotic twinning varies widely by population; monozygotic twinning occurs at a relatively constant rate across populations. ^[5]
• No modifiable maternal factor is known for monozygotic twinning itself, and there is no reliable way to prevent it.

A monochorionic placenta almost always contains vascular anastomoses between the two umbilical circulations: artery-to-artery and vein-to-vein connections on the placental surface, and deep artery-to-vein connections within the cotyledons. ^[2] These connections are the engine of the monochorionic complications.

Twin-twin transfusion syndrome (TTTS) is an imbalance across arteriovenous anastomoses that runs one way. Blood leaves the donor twin faster than it returns, so the donor becomes oliguric, oligohydramniotic and growth-restricted, while the recipient becomes polyuric, polyhydramniotic and volume overloaded, with risk of hydrops and cardiac dysfunction. ^[2][4] It is a continuum rather than a switch: mildly discordant amniotic fluid may progress to severe disease.

Selective fetal growth restriction (sFGR) occurs when one twin gets a smaller share of the placenta and grows poorly, with a large estimated fetal weight (EFW) discordance. Twin anaemia-polycythaemia sequence (TAPS) is a slow, chronic form of unequal red cell sharing across small anastomoses, producing severe anaemia in one twin and polycythaemia in the other without the gross fluid discordance of TTTS. Twin reversed arterial perfusion (TRAP) sequence occurs when one twin, usually acardiac and anencephalic, receives reversed blood flow from the healthy “pump” twin via an artery-to-artery anastomosis. ^[2][4]

Monochorionic monoamniotic twins share one sac, so the cords can entangle: monoamniotic twinning occurs in about 3% of monochorionic twin pregnancies (about 0.02% of all pregnancies) and carries a high risk of stillbirth from cord entanglement. ^[4]

The monochorionic complications (TTTS, TAPS, TRAP sequence) together occur in roughly 15% of monochorionic twin pregnancies. ^[4]

• Maternal: higher rates of anaemia (iron and folate demand for two), hypertension and pre-eclampsia, gestational diabetes, hyperemesis, and postpartum haemorrhage from an over-distended uterus.
• Fetal/neonatal: congenital anomalies are more common in multiple than singleton pregnancies; growth discordance; preterm birth; perinatal loss; in monochorionic pregnancy, brain injury in a surviving twin after the death of one twin, because of acute transfusion through the shared vessels.

Many women are asymptomatic until the routine dating scan. A multiple pregnancy should be suspected when:

• the uterus is larger than expected for dates, or fundal height plots above the 90th centile;
• fetal movements feel excessive or very frequent early in pregnancy;
• hyperemesis gravidarum is marked (higher human chorionic gonadotropin);
• routine auscultation is difficult because more than one fetal heart is heard;
• there is polyhydramnios.

History points that change management: dating (last menstrual period, IVF dates), a previous multiple pregnancy, family history of dizygotic twinning, and symptoms of pre-eclampsia (headache, visual change, right upper quadrant pain) or preterm labour.

Examination: inspection of fundal height and lie; abdominal palpation for more than two poles and multiple small parts; careful auscultation (a fetal Doppler confirms separate cardiac activity and rates). Digital vaginal examination is reserved for specific indications. Fetal presentation must be documented at every late-pregnancy visit because it changes delivery planning.

Red flags that should trigger same-day specialist review: sudden uterine growth or pain with polyhydramnios (TTTS); bleeding; contractions before 37 weeks; decreased fetal movements; severe headache or visual disturbance; shortness of breath at rest.

Before an ultrasound confirms the diagnosis, several conditions can mimic a large-for-dates uterus. ^[1]

Chorionicity is determined at the booking scan and should be rechecked if the woman presents late. NICE, ISUOG and RCOG all agree: a monochorionic pregnancy is managed as monochorionic until proven otherwise if doubt remains. ^[1][2][5]

TTTS is the most dangerous monochorionic complication. It is diagnosed on ultrasound using the amniotic fluid discordance thresholds above, with Doppler deterioration in advancing stages. ^[1] Treatment is by a fetal medicine specialist: expectant management or amnioreduction for very mild disease, and fetoscopic laser photocoagulation of the placental anastomoses when the disease is significant, because laser removes the cause rather than draining fluid. ^[2][4] Weekly MCA-PSV surveillance is added after laser or in suspected TAPS. ^[1]

All women with a twin or triplet pregnancy should be managed by a nominated multidisciplinary core team (specialist obstetricians, midwives and experienced sonographers), with an enhanced team (mental health, physiotherapy, infant feeding, dietetics) brought in as needed. ^[1] Monochorionic and shared-amnion pregnancies are referred to a tertiary fetal medicine centre. ^[1][2]

NICE sets out a schedule of specialist appointments: at least 8 for an uncomplicated dichorionic diamniotic twin pregnancy and at least 11 for an uncomplicated monochorionic diamniotic twin pregnancy, combining visits with scans at the gestations listed in the recommendations. ^[1] Monochorionic twins are scanned every 2 weeks from 16 weeks until birth; dichorionic twins are scanned in the first trimester, at the anomaly scan, and every 4 weeks thereafter. ^[1][5] Growth scans allow 30 minutes and anomaly scans 45 minutes. ^[1]

• Progesterone: NICE now recommends a single cervical length scan at 16 to 20 weeks in twin and triplet pregnancy. If the cervix is 25 mm or less, offer progesterone 200 mg vaginal capsules nightly until 34 weeks or birth. If a short cervix is found incidentally at 20 to 24 weeks, offer the same. ^[1]
• Do not use: intramuscular progesterone, cervical cerclage, arabin pessary, bed rest or routine oral tocolytics in twins, since these have not shown benefit and may cause harm. ^[1]
• Corticosteroids: only targeted courses, not routine or repeated untargeted courses; there is no benefit from untargeted administration in twin pregnancy. ^[1]
• Aspirin: advise women with a twin or triplet pregnancy to take low-dose aspirin daily from 12 weeks until birth if they have two or more of the risk factors listed in NICE's hypertension in pregnancy guidance. ^[1] ACOG/SMFM likewise includes low-dose aspirin in monochorionic antenatal care for pre-eclampsia prevention. ^[3][4]

• Hypertensive screening: measure blood pressure and test urine for proteinuria at every antenatal visit. ^[1]
• Growth discordance: see the cut-offs in the diagnosis table above. A 20% or more EFW discordance with any EFW under the 10th centile means weekly monitoring with umbilical artery Doppler; a 25% or more discordance with any EFW under the 10th centile means referral. ^[1]
• Fetofetal transfusion sequence: diagnosed at DVP thresholds above; the woman is referred to a tertiary fetal medicine centre for assessment, where laser photocoagulation is the standard definitive treatment for significant TTTS. ^[1][2][4]
• TAPS and TRAP sequence: managed in tertiary fetal medicine centres (MCA-PSV surveillance, intrauterine transfusion or laser where appropriate). ^[1][2]

Timing (NICE NG137): 37 weeks for an uncomplicated dichorionic diamniotic twin pregnancy; 36 weeks for an uncomplicated monochorionic diamniotic twin pregnancy (after corticosteroids have been considered); 32+0 to 33+6 weeks for an uncomplicated monochorionic monoamniotic twin pregnancy; 35 weeks for an uncomplicated trichorionic triamniotic or dichorionic triamniotic triplet pregnancy; individual assessment for a monochorionic triamniotic triplet pregnancy or any triplet pregnancy with a shared amnion. ^[1]

Comparison with other guidance:

• ISUOG considers the optimal delivery window for uncomplicated dichorionic twins to be 37+0 to 37+6 weeks and for uncomplicated monochorionic twins 36+0 to 36+6 weeks. ^[5]
• ACOG/SMFM recommend delivery between 34 0/7 and 37 6/7 weeks for monochorionic diamniotic twins, and planned birth by the dates agreed with the couple in dichorionic pregnancies. ^[3][4]
• NICE and ACOG/SMFM therefore agree on the principle (planned, early-term delivery) and differ only by a few days. ^[1][3][4][5]

Mode of birth: NICE recommends that for an uncomplicated twin pregnancy planned vaginal birth and planned caesarean section are both safe when the pregnancy is past 32 weeks, there are no obstetric contraindications, the first twin is cephalic and there is no significant size discordance. More than one third of women who plan a vaginal birth go on to have a caesarean section. ^[1] Offer caesarean section if the first twin is not cephalic, for a monochorionic monoamniotic twin pregnancy, and for all triplet pregnancies. ^[1] In dichorionic or monochorionic diamniotic twins with a cephalic first twin, vaginal birth may be considered from 32 weeks if an experienced obstetrician is available. ^[4]

Offer continuous cardiotocography to women in established labour in a twin pregnancy over 26 weeks; perform a portable ultrasound at the start of labour to confirm presentation and locate both fetal hearts; dual-channel monitoring should be used and the two traces must belong to different babies. ^[1] Active management of the third stage is standard, with uterotonic drugs and attention to postpartum haemorrhage.

Maternal morbidity is higher in multiple than singleton pregnancy: anaemia (iron and folate consumption), pre-eclampsia and gestational hypertension, gestational diabetes, hyperemesis gravidarum, venous thromboembolism, and postpartum haemorrhage from uterine overdistension. ^[3][5]

Fetal and neonatal complications include congenital structural anomalies (more common in multiple than singleton pregnancies) ^[3], selective fetal growth restriction, growth discordance, preterm birth and its consequences, single fetal death with neurological injury to the survivor in monochorionic pregnancies, and perinatal loss. ^[5]

Complications unique to monochorionic placentation (about 15% of monochorionic pregnancies): TTTS, TAPS, TRAP sequence, and (in monoamniotic pairs) cord entanglement with stillbirth. ^[2][4]

Triplet and higher-order pregnancies multiply every one of these risks; iatrogenic prematurity is frequent, and admission to a neonatal unit is common. NICE notes that about 75 of 100 triplet pregnancies deliver before 35 weeks. ^[1]

Uncomplicated dichorionic and monochorionic twin pregnancies managed by current surveillance schedules have perinatal outcomes approaching those of singletons in many respects, but prematurity, growth discordance and stillbirth risk remain higher. ^[5] About 10% of twin pregnancies deliver before 32 weeks, versus about 1% of singleton pregnancies, which drives most of the excess neonatal mortality and long-term morbidity. ^[5] Once complications such as TTTS are treated with laser, survival has improved substantially in modern series, although neurodevelopmental follow-up of survivors is still advised.

• Avoid iatrogenic multiples: single embryo transfer in assisted reproduction is the most effective prevention of high-order multiple pregnancy. ^[5]
• Chorionicity screening at 11 to 13+6 weeks is the first and most important step; scanning the whole membrane and noting the lambda or T sign. ^[1][5]
• Aneuploidy screening with first-trimester combined testing or non-invasive prenatal testing, with counselling about the higher chance of invasive testing in twins. ^[5]
• Structural anomaly screening at about 20 weeks, allowing about 45 minutes. ^[1]
• Folic acid and iron: be alert to the higher incidence of anaemia in multiple pregnancy; a full blood count at 20 to 24 weeks and again at 28 weeks. ^[1]
• Low-dose aspirin from 12 weeks for women with two or more hypertension risk factors. ^[1][3][4]
• Preterm prevention with progesterone as above, and education about symptoms of preterm labour from booking. ^[1]

Pregnancy (the index pregnancy). Almost all the management in this page concerns the pregnant woman carrying multiples; twin and triplet pregnancy should be considered a high-risk pregnancy with scheduled specialist review. ^[1]

Children of the pregnancy. Monozygotic twins, especially after TTTS or laser, need neonatal assessment and may need brain imaging if there has been fetal compromise. Single-fetal demise in a monochorionic pregnancy leaves a surviving twin at risk of neurological injury, and delivery timing is individualised by fetal medicine. ^[1][2]

Older adults. Women of advanced maternal age are more likely to conceive twins, and the risk of hypertension, gestational diabetes, anaemia and preterm delivery rises with age; counselling about screening and early delivery should be tailored. No specific age cut-off is given in the guidelines cited. ^[1][5]

Low-resource settings. A late or uncertain first scan is common. If chorionicity cannot be determined, manage the pregnancy as monochorionic until proven otherwise, because this is the safer default. ^[1][5] Fetal Doppler on a basic machine can count two hearts and confirm viability. Where fetoscopic laser is unavailable, refer early to a tertiary centre; if referral is impossible, amnioreduction and intensive monitoring remain the fallback. ^[2] Educate women to recognise preterm labour and decreased fetal movements.

The conditions most often confused with a multiple pregnancy in the antenatal period are listed below. None of them shares the shared-placenta physiology of monochorionic twinning. ^[1]

• Multiple gestation is defined as more than one fetus; classify by chorionicity (placentas) and amnionicity (sacs), not zygosity.
• Monochorionic twins share a placenta and account for about 20 to 30% of twin pregnancies; they carry the unique risks of TTTS, TAPS, TRAP sequence and cord entanglement. ^[2][4]
• About 15% of monochorionic pregnancies develop TTTS, TAPS or TRAP sequence. ^[4]
• Multiple pregnancy is the leading cause of preterm birth: up to 60% of multiple pregnancies deliver before 37 weeks, and about 10% of twins deliver before 32 weeks versus about 1% of singletons. ^[5]
• Determine chorionicity at 11 to 13+6 weeks; if in doubt, manage as monochorionic. ^[1][5]
• Use lambda/twin peak (dichorionic) versus T sign (monochorionic) on ultrasound. ^[5]
• Monochorionic twins need scans every 2 weeks from 16 weeks; dichorionic twins every 4 weeks from 20 weeks. ^[1][5]
• TTTS is diagnosed at donor DVP under 2 cm with recipient DVP over 8 cm before 20 weeks (over 10 cm from 20 weeks). ^[1]
• An EFW discordance of 20% or more with any EFW under the 10th centile needs weekly monitoring; 25% or more means referral. ^[1]
• Offer progesterone 200 mg vaginally nightly for a cervical length of 25 mm or less; do not use cerclage, pessary, bed rest or routine oral tocolytics. ^[1]
• Give low-dose aspirin daily from 12 weeks to women with two or more hypertension risk factors. ^[1][3][4]
• Plan delivery at 37 weeks for uncomplicated dichorionic twins, 36 weeks for monochorionic diamniotic twins (after corticosteroids considered), 35 weeks for uncomplicated triamniotic or dichorionic triplets, and 32+0 to 33+6 weeks for monochorionic monoamniotic twins. ^[1]
• Continuous cardiotocography is recommended in labour in twin pregnancy beyond 26 weeks. ^[1]
• Monoamniotic twins occur in about 3% of monochorionic pregnancies and carry a high risk of cord entanglement; plan caesarean delivery. ^[4]
• WHO provides a global antenatal care framework, including a minimum of eight antenatal contacts; the multiple-pregnancy-specific detail comes from NICE, RCOG, ACOG/SMFM and ISUOG. ^[6]

Related (not yet linked here as topics are added): hypertensive disorders in pregnancy, preterm birth and labour, antenatal screening, fetal growth restriction, postpartum haemorrhage, ultrasound in obstetrics.