Why twins arrive early
Around half of all twins are born before 37 weeks, and the average twin pregnancy lasts about 35–36 weeks. The uterus becomes very stretched, the risk of complications such as pre-eclampsia rises, and the placenta(s) may not keep up with the needs of two growing babies. For these reasons, obstetricians usually plan delivery before 40 weeks, even when everything is going well.
Types of twins: chorionicity
The single most important fact about a twin pregnancy is its chorionicity – whether the babies share a placenta – best determined on a scan before 14 weeks.
| Type | Placenta / sac | How common | Typical planned birth* |
|---|---|---|---|
| DCDA (dichorionic diamniotic) | Two placentas, two sacs | ~70–80% of twins; all non-identical twins | 38w 0d – 38w 6d |
| MCDA (monochorionic diamniotic) | One placenta, two sacs | Most identical twins | 34w 0d – 37w 6d |
| MCMA (monochorionic monoamniotic) | One placenta, one sac | Rare (~1%) | 32w 0d – 34w 0d |
*For uncomplicated pregnancies, based on ACOG guidance. UK (NICE) guidance is similar, suggesting 37 weeks for DCDA and 36 weeks for MCDA twins. Your obstetrician will personalise the plan.
Extra care in twin pregnancy
- More frequent scans – every 2 weeks from 16 weeks for monochorionic twins to watch for twin-to-twin transfusion syndrome (TTTS), and every 4 weeks for DCDA twins.
- Higher needs for iron, folic acid, calcium and protein; anaemia is more common.
- Closer blood-pressure and blood-sugar monitoring.
- Delivery in a hospital with a neonatal intensive care unit (NICU) is strongly advised.
With twins, seek care promptly for regular tightenings before 37 weeks, fluid leaking, bleeding, severe headache, or a sudden increase in bump size or breathlessness.
Frequently asked questions
How many weeks are twins usually born?
On average around 35–36 weeks. Planned delivery for uncomplicated twins is usually between 36 and 38 weeks, depending on type.
Is the due date different for twins?
The due date (40 weeks) is calculated the same way, but twins are very unlikely to reach it. Doctors use the due date to track gestational age and plan an earlier delivery.
How do I know if my twins share a placenta?
An ultrasound scan in the first trimester (ideally before 14 weeks) shows whether there is a thick dividing membrane (lambda sign – two placentas) or a thin one (T-sign – shared placenta).
Can twins be born naturally?
Yes. If the first twin is head-down and there are no other concerns, vaginal birth is often possible. Your obstetrician will discuss the safest option.
How much weight should I gain with twins?
For a normal pre-pregnancy BMI, 17–25 kg. Try our weight gain calculator and tick ‘I am expecting twins’.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- ACOG. Committee Opinion No. 831: Medically Indicated Late-Preterm and Early-Term Deliveries (2021).
- NICE Guideline NG137. Twin and triplet pregnancy (2019).
- Institute of Medicine & National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines (2009). National Academies Press.