Clinical overview
Multiple pregnancy — twins, triplets and higher orders — is one of the highest-risk states in routine obstetrics, and the registrar must be able to manage it confidently because the rising tide of assisted reproduction and advancing maternal age has pushed twin rates above their historical baseline. The natural dizygotic twinning rate is influenced by race, with sub-Saharan African populations carrying among the highest spontaneous rates in the world, so in South African practice twins are not a rarity but a regular feature of the antenatal clinic and the labour ward. Every adverse outcome in obstetrics is amplified in the multiple: preterm birth, pre-eclampsia, gestational diabetes, growth restriction, antepartum and postpartum haemorrhage, malpresentation and operative delivery are all more common. Perinatal mortality is several-fold higher than in singletons, and most of that excess is driven by prematurity and by the chorionicity-specific complications of monochorionic placentation.
The single most important determination after diagnosing a twin pregnancy is chorionicity — not zygosity. Chorionicity dictates the entire surveillance schedule, the risk of unique complications such as twin-to-twin transfusion syndrome (TTTS), and the timing and mode of delivery. The objective here is the uncomplicated multiple pregnancy and its diagnosis and delivery, including vaginal birth; but "uncomplicated" can only be claimed after chorionicity is fixed and the chorionicity-driven complications have been actively screened for and excluded. This chapter sits beside High-risk pregnancy factors and the obstetric-emergency chapters, and the registrar should read it with Antenatal booking, Gestational age assessment and Complicated labour alongside.
Core knowledge
Zygosity and chorionicity
- Dizygotic (DZ) twins arise from two separately fertilised ova: always two placentas, two chorions, two amnions — dichorionic diamniotic (DCDA). They are no more genetically alike than any siblings and may be different sexes.
- Monozygotic (MZ) twins arise from a single zygote that divides. The resulting chorionicity depends on when the split occurs:
- Split at days 0–3 (morula/pre-blastocyst): DCDA (about a third of MZ).
- Split at days 4–8 (blastocyst): monochorionic diamniotic (MCDA) — the commonest MZ form.
- Split at days 8–13 (after amnion forms): monochorionic monoamniotic (MCMA).
- Split after day 13: incomplete — conjoined twins.
All monochorionic twins are monozygotic; the placental vascular anastomoses they share are the source of TTTS, twin anaemia–polycythaemia sequence (TAPS), selective fetal growth restriction with abnormal Doppler, and the catastrophic risk to the survivor after a single intrauterine death. Dichorionic twins do not share circulation and so escape these specific syndromes, though they remain at raised risk of prematurity, growth restriction and pre-eclampsia.
Figure J23.1 — Chorionicity pathway for twins, showing dizygotic DCDA pregnancy, monozygotic split timing, lambda/T-sign ultrasound clues, and monochorionic risk syndromes.
Why multiples are high risk
The shared maternal physiology is stretched: plasma volume expansion and cardiac output are greater, anaemia is more common, and uterine over-distension predisposes to preterm labour and to uterine atony with postpartum haemorrhage. Spontaneous preterm birth before 37 weeks affects roughly six in ten twin pregnancies, and the median gestation at birth falls as the fetal number rises. Pre-eclampsia, gestational diabetes and obstetric cholestasis are all over-represented. Growth restriction may be of both fetuses or, classically in monochorionic pairs, selective. These are the reasons a twin pregnancy is booked and followed in a higher level of care.
Assessment
Diagnosis
Multiple pregnancy is diagnosed by ultrasound, ideally at the first-trimester scan. Clinical suspicion (uterus large for dates, two fetal poles, more than two fetal heart tones, hyperemesis) should always be confirmed sonographically. The NICE multiple-pregnancy guidance (NG137) and the SA National Integrated Maternal and Perinatal Care Guideline (NDoH, 2024) both make early dating-and-chorionicity ultrasound the cornerstone of care.
Determining chorionicity is most reliable between 11+0 and 13+6 weeks and should be documented in the notes and on the report:
- Two separate placental masses → dichorionic.
- The membrane–placenta junction:
- Lambda (λ) / "twin peak" sign — a triangular wedge of placental tissue extending into the inter-twin membrane base → dichorionic.
- T sign — the inter-twin membrane meets the placenta at a right angle with no wedge → monochorionic.
- Inter-twin membrane thickness — thin (two layers) suggests monochorionic; thick (three or four layers) suggests dichorionic — a softer, later sign.
- Discordant fetal sex proves dichorionicity.