Synopsis
Use chorionicity and amnionicity to organise multiple-pregnancy care, recognise shared-placenta complications and plan surveillance, preterm prevention and birth for each fetus.
- Determine chorionicity and amnionicity early; they guide risk and surveillance more directly than whether twins are genetically identical.
- Use consistent fetal labels throughout every scan and report so measurements and interventions are attributed to the same fetus.
- Monochorionic pregnancies need ultrasound surveillance every 14 days from 16 weeks for transfusion-related and growth complications.
- Calculate weight discordance as the difference between larger and smaller estimated weight divided by the larger weight, multiplied by 100.
- Increase surveillance for at least 20% discordance or any fetus below the tenth centile; at least 25% plus a fetus below the tenth warrants tertiary review.
- For uncomplicated twins, offer planned birth at 37 weeks for DCDA, 36 weeks for MCDA and 32+0 to 33+6 weeks for MCMA.
Key red flags
A monochorionic donor sac DVP below 2 cm with recipient DVP above 8 cm before 20+0 weeks or above 10 cm from 20+0 weeks requires referral for suspected TTTS.
Rapid painful abdominal enlargement, breathlessness, reduced movements or preterm labour symptoms in a multiple pregnancy warrants immediate maternity review.
Single fetal death in a monochorionic pregnancy needs urgent specialist assessment of the survivor because shared vessels create additional neurological and circulatory risks.