Multiple pregnancy refers to a pregnancy with more than one fetus. The incidence of multiple pregnancies increased with the development of fertility treatment.
There are some key definitions to become familiar with relating to twin and multiple pregnancy:
- Monozygotic: identical twins (from a single zygote)
- Dizygotic: non-identical (from two different zygotes)
- Monoamniotic: single amniotic sac
- Diamniotic: two separate amniotic sacs
- Monochorionic: share a single placenta
- Dichorionic: two separate placentas
The best outcomes are with diamniotic, dichorionic twin pregnancies, as each fetus has their own nutrient supply.
Multiple pregnancy is usually diagnosed on the booking ultrasound scan. Ultrasound is also used to determine the:
- Gestational age
- Number of placentas (chorionicity) and amniotic sacs (amnionicity)
- Risk of Down’s syndrome (as part of the combined test)
When determining the type of twins using an ultrasound scan:
- Dichorionic diamniotic twins have a membrane between the twins, with a lambda sign or twin peak sign
- Monochorionic diamniotic twins have a membrane between the twins, with a T sign
- Monochorionic monoamniotic twins have no membrane separating the twins
The lambda sign, or twin peak sign, refers to a triangular appearance where the membrane between the twins meets the chorion, as the chorion blends partially into the membrane. This indicates a dichorionic twin pregnancy (separate placentas).
The T sign refers to where the membrane between the twins abruptly meets the chorion, giving a T appearance. This indicates a monochorionic twin pregnancy (single placenta).
Risks to the mother:
- Spontaneous preterm birth
- Instrumental delivery or caesarean
- Postpartum haemorrhage
Risks to the fetuses and neonates:
- Fetal growth restriction
- Twin-twin transfusion syndrome
- Twin anaemia polycythaemia sequence
- Congenital abnormalities
Twin-Twin Transfusion Syndrome
Twin-twin transfusion syndrome occurs when the fetuses share a placenta. It is called feto-fetal transfusion syndrome in pregnancies with more than two fetuses.
When there is a connection between the blood supplies of the two fetuses, one fetus (the recipient) may receive the majority of the blood from the placenta, while the other fetus (the donor) is starved of blood. The recipient gets the majority of the blood, and can become fluid overloaded, with heart failure and polyhydramnios. The donor has growth restriction, anaemia and oligohydramnios. There will be a discrepancy between the size of the fetuses.
Women with twin-twin transfusion syndrome need to be referred to a tertiary specialist fetal medicine centre. In severe cases, laser treatment may be used to destroy the connection between the two blood supplies.
Twin Anaemia Polycythaemia Sequence
Twin anaemia polycythaemia sequence is similar to twin-twin transfusion syndrome, but less acute. One twin becomes anaemic whilst the other develops polycythaemia (raised haemoglobin).
The NICE guidelines (2019) on multiple pregnancy advise about additional management for multiple pregnancies. A specialist multiple pregnancy obstetric team manages women with a multiple pregnancy.
Women with multiple pregnancies require additional monitoring for anaemia, with a full blood count at:
- Booking clinic
- 20 weeks gestation
- 28 weeks gestation
Additional ultrasound scans are required in multiple pregnancy to monitor for fetal growth restriction, unequal growth and twin-twin transfusion syndrome:
- 2 weekly scans from 16 weeks for monochorionic twins
- 4 weekly scans from 20 weeks for dichorionic twins
Planned birth is offered between:
- 32 and 33 + 6 weeks for uncomplicated monochorionic monoamniotic twins
- 36 and 36 + 6 weeks for uncomplicated monochorionic diamniotic twins
- 37 and 37 + 6 weeks for uncomplicated dichorionic diamniotic twins
- Before 35 + 6 weeks for triplets
Waiting beyond these dates is associated with an increased risk of fetal death. The timing of birth when there are complications is assessed on an individual basis. Corticosteroids are given before delivery to help mature the lungs.
Monoamniotic twins require elective caesarean section at between 32 and 33 + 6 weeks.
Diamniotic twins (aim to deliver between 37 and 37 + 6 weeks):
- Vaginal delivery is possible when the first baby has a cephalic presentation (head first)
- Caesarean section may be required for the second baby after successful birth of the first baby
- Elective caesarean is advised when the presenting twin is not cephalic presentation
Last updated September 2020