Obstetrics and Gynaecology chapters
Multiple pregnancy: twins, chorionicity and twin-to-twin transfusion
Twin and higher-order pregnancies: zygosity and chorionicity, the lambda and T signs, maternal and fetal risks, twin-to-twin transfusion syndrome, antenatal care and how twins are delivered.
- Twins are dizygotic (70%), always dichorionic diamniotic, or monozygotic (30%), whose placentation depends on when the embryo splits. Chorionicity decides the risk and is best seen at 11-14 weeks: lambda sign = dichorionic, T sign = monochorionic.
- Almost every complication is commoner: preterm birth (the main killer), FGR, pre-eclampsia, anaemia, APH and PPH. Twin-to-twin transfusion affects about 10% of monochorionic diamniotic twins and is treated by fetoscopic laser.
- Plan delivery: dichorionic twins from 37 weeks, monochorionic from 36 weeks, monoamniotic by caesarean at 32-34 weeks; vaginal birth is reasonable if twin one is cephalic.
On this page
Overview
Multiple pregnancy makes up about 3% of live births and is increasing, mainly because of assisted conception and older maternal age. 97-99% are twins. Multiple pregnancies carry a higher risk of almost every complication of pregnancy; the exceptions are macrosomia and post-dates pregnancy. The single most important variable is chorionicity, not zygosity.
Classification
Multiple pregnancies are described by:
- Number of fetuses: twins, triplets and so on.
- Zygosity: number of fertilised eggs.
- Chorionicity: number of placentas.
- Amnionicity: number of amniotic sacs.
Dizygotic (non-identical) twins: about 70%
- Two eggs released and fertilised separately.
- Always dichorionic diamniotic (DCDA); placentas may fuse but stay functionally separate.
- Same or different sex.
- Risk factors: ovulation induction (clomiphene, gonadotrophins), IVF with more than one embryo, older maternal age, and ethnicity (highest in parts of Africa such as Nigeria; lowest in Japan).
Monozygotic (identical) twins: about 30%
The placentation depends on when the single embryo splits:
| Time of split after fertilisation | Result |
|---|---|
| Very early (shortly after fertilisation) | Dichorionic diamniotic |
| Days 4-8 | Monochorionic diamniotic (MCDA), the commonest monozygotic type |
| Days 8-12 | Monochorionic monoamniotic (MCMA), about 1% of twins |
| After day 13 | Conjoined twins |
"The later the split, the more they share."
Same-sex twins with a fused-looking placenta can still be dizygotic. All dizygotic twins are dichorionic, but not all dichorionic twins are dizygotic.
Diagnosis and chorionicity
- Clues: uterus large for dates, severe vomiting, high hCG, two fetal hearts, many fetal parts; history of fertility treatment.
- Ultrasound confirms the diagnosis.
- Chorionicity is best determined in the first trimester: the scan is done when CRL measures 45-84 mm (11+0 to 13+6 weeks).
| Sign | Appearance | Meaning |
|---|---|---|
| Lambda (twin peak) sign | Wedge of placental tissue extending into the base of the dividing membrane; thick membrane (amnion with chorion on each side) | Dichorionic |
| T sign | Thin membrane meeting the placenta at a right angle (two layers of amnion only) | Monochorionic |
| No dividing membrane | Single sac | Monoamniotic |
The same scan dates the pregnancy and screens for Down syndrome. Fetuses are mapped (e.g. "twin 2, maternal upper right") so later scans stay consistent.
Complications
Maternal
- Exaggerated minor symptoms: nausea and vomiting (hyperemesis), heartburn, backache.
- Anaemia (greater iron and folate demand): FBC at 20 and 28 weeks, routine supplements.
- Pre-eclampsia and hypertension.
- Gestational diabetes.
- Antepartum haemorrhage (praevia from a large placenta; abruption).
- Thromboembolism.
- Postpartum haemorrhage (overdistended uterus).
- More operative deliveries; worse outcome in women with heart disease.
Fetal
- Preterm birth: the commonest and most serious problem. About 60% of twins are born before 37 weeks; uterine overdistension is the main mechanism. Triplets deliver at about 34 weeks on average.
- Fetal growth restriction: each dichorionic twin has twice the singleton risk; monochorionic twins almost double that again. A weight difference of 25% or more is significant.
- Congenital anomalies: higher in monozygotic twins (two to three times).
- Stillbirth: about 12 per 1,000 twin births versus about 5 per 1,000 singletons.
- Perinatal mortality: about 30 per 1,000 for monochorionic versus 3.8 per 1,000 for dichorionic twins.
- Cerebral palsy is more common.
- Vanishing twin: early loss of one twin, seen in up to a quarter of twins on early scans.
- Cord entanglement in monoamniotic twins.
Prematurity is the main reason twins have higher perinatal mortality and NICU admissions.
Death of one twin
- Dichorionic: the survivor usually does well; labour may start early.
- Monochorionic: placental vessel connections let blood shift suddenly from the living to the dead twin, causing death or brain damage of the survivor in up to 30%. Maternal DIC is described but very rare.
Complications unique to monochorionic twins
Twin-to-twin transfusion syndrome (TTTS)
- Nearly all monochorionic placentas have vascular connections. TTTS develops when these are unbalanced, with more arteriovenous connections running one way.
- Occurs in about 10% of MCDA and 5% of MCMA twins. Arterioarterial connections are protective.
- Donor twin: hypovolaemic, little urine, oligohydramnios, small or invisible bladder, growth restriction.
- Recipient twin: hypervolaemic, polyuria and polyhydramnios, large bladder, heart failure, hydrops.
Ultrasound criteria: single placenta, same sex, deepest pool under 2 cm in one sac and over 8 cm in the other, discordant bladders, cardiac compromise.
Quintero staging
| Stage | Finding |
|---|---|
| I | Oligo-polyhydramnios; donor bladder still visible; normal Dopplers |
| II | Donor bladder not seen; Dopplers normal |
| III | Abnormal Dopplers (absent or reversed umbilical end-diastolic flow, reversed ductus venosus a-wave, pulsatile umbilical vein) |
| IV | Hydrops in one or both |
| V | Death of one or both |
Treatment: refer to a fetal medicine centre. Fetoscopic laser ablation of the connecting vessels is the definitive treatment for stage II or above between 16 and 26 weeks; it doubles survival compared with draining fluid (amnioreduction). After 26 weeks, delivery may be considered. Other options: amnioreduction, septostomy, selective feticide.
Surveillance: monochorionic twins are scanned every 2 weeks from 16 weeks to at least 24 weeks, then for growth.
Twin anaemia-polycythaemia sequence (TAPS)
A slower form of transfusion through tiny connections: one twin becomes anaemic, the other polycythaemic, without the fluid discrepancy of TTTS.
Monoamniotic twins
- About 1 in 10,000 pregnancies; highest risk type.
- Cord entanglement causes sudden death; more congenital anomalies.
- Admission from about 28 weeks with frequent CTGs in many units; caesarean at 32-34 weeks.
TTTS needs a shared placenta. It cannot occur in dichorionic twins.
Antenatal care
- Care by a specialist multidisciplinary twin team; monoamniotic and complicated twins at a tertiary fetal medicine unit.
- Dichorionic: at least 8 antenatal visits; MCDA: at least 9; triplets: at least 11.
- Serial growth scans from 20 weeks, at most 4-weekly (fortnightly for monochorionic twins). SFH and palpation are not reliable for growth in twins.
- Iron and folic acid supplements; screen for pre-eclampsia and gestational diabetes.
- Untargeted steroids and cervical cerclage do not prevent preterm birth in twins.
Timing and mode of delivery
| Type | Planned delivery (uncomplicated) |
|---|---|
| DCDA | From 37 weeks |
| MCDA | From 36 weeks, after a course of steroids |
| MCMA | Caesarean section at 32-34 weeks |
| Triplets | Not beyond 36 weeks |
Continuing twins beyond 38 weeks raises the risk of stillbirth.
Labour
- Vaginal birth is reasonable for uncomplicated DCDA (and MCDA) twins if twin one is cephalic.
- Continuous fetal monitoring of both twins; senior obstetrician, two neonatal teams and resuscitation trolleys, anaesthetist aware.
- Early epidural helps if manipulation of twin two is needed.
- After twin one: check the lie of twin two by abdominal palpation and portable ultrasound.
- Longitudinal: wait for descent, then rupture membranes; an oxytocin infusion prepared in advance may be needed if contractions fade.
- Breech: rupture membranes once the breech is fixed; breech extraction if needed.
- Transverse: external cephalic version (success over 70%) or internal podalic version by an experienced operator.
- About 4% need an emergency caesarean for twin two after a vaginal birth of twin one.
- Oxytocin infusion after delivery to prevent PPH.
Twin two is non-vertex in about 40% of twin labours; this alone is not a reason for caesarean.
Expect: dizygotic 70% versus monozygotic 30%; timing of the split (day 4-8 MCDA, day 8-12 MCMA, after 13 conjoined); chorionicity at 11-14 weeks with lambda (dichorionic) and T (monochorionic) signs; clomiphene and IVF as causes; prematurity as the main cause of death; pre-eclampsia, anaemia, APH and PPH; TTTS only in monochorionic twins, donor oligohydramnios and recipient polyhydramnios, Quintero staging and laser treatment; death of one monochorionic twin; delivery at 37, 36 and 32-34 weeks; vaginal birth if twin one is cephalic; internal podalic version.
Quick revision
- About 3% of births; driven by IVF, ovulation induction and older age.
- Dizygotic 70% (always DCDA); monozygotic 30%.
- Split day 4-8 MCDA; 8-12 MCMA; after 13 conjoined.
- Lambda sign: dichorionic; T sign: monochorionic; scan at 11-14 weeks.
- Prematurity is the main killer; FGR, pre-eclampsia, anaemia, APH, PPH all rise.
- TTTS: monochorionic only; laser at 16-26 weeks for stage II or above.
- Single monochorionic death: up to 30% death or damage of the co-twin.
- Deliver DCDA 37, MCDA 36, MCMA 32-34 weeks by caesarean.
Written by The House Job from standard references (Obstetrics by Ten Teachers; Gynaecology by Ten Teachers).
FAQ
Are monozygotic twins always monochorionic?
No. If the embryo splits very soon after fertilisation, monozygotic twins get separate placentas and are dichorionic diamniotic.
Why does TTTS only happen in monochorionic twins?
It depends on blood vessel connections across a shared placenta; dichorionic twins have separate placental circulations.
Why does twin pregnancy cause anaemia?
Plasma volume rises even more than in a singleton and two fetuses draw on maternal iron and folate, so dilution and deficiency are both more likely.
What is the main risk after twin one is born vaginally?
Twin two may change lie or develop distress, and contractions may weaken, so the team must check its lie quickly and be ready for version, assisted delivery or caesarean.