Twin and Triplet Pregnancies
NICE guideline [NG137] Twin and triplet pregnancy. Last updated: Apr 2024.
RCOG Monochorionic Twin Pregnancy, Management – 2024 Partial Update (Green-top Guideline No. 51). Last reviewed: Feb 2024.
RCOG Patient information Multiple pregnancy: having more than one baby. Last reviewed: 2023.
Epidemiology
Twins or triplets occur in approximately 1 in 60 pregnancies.
The incidence of multiple pregnancies has risen over the past 30 years, primarily driven by:
- Use of ARTs (e.g. IVF)
- Advanced maternal age (women are deferring pregnancy to an older age)
Identical vs Non-Identical Twins
Twins or triples can be identical or non-identical:
| Type | Mechanism | Clinical phenotype |
|---|---|---|
| Identical (monozygotic) twin | ONE egg fertilised by ONE sperm, then splits into TWO after fertilisation | Since they originate from the same egg → identical genes
*Monochorionic pregnancies are always identical (monozygotic) twins |
| Non-identical (dizygotic) twin | DIFFERENT egg fertilised by DIFFERENT sperm | Different egg → different genes
|
Types of Twin Pregnancy
There are 3 main types of twin pregnancies:
| Type | Prevalence | Description | Identical vs non-identical |
|---|---|---|---|
| Dichorionic diamniotic (DCDA) | Most common (~80%) | Each baby has its own placenta
Each baby has its own amniotic sac |
These babies are more likely to be non-identical (dizygotic) |
| Monochorionic diamniotic (MCDA) | ~15% | Both babies share a single placenta
Each baby has its own amniotic sac |
These babies are always identical (monozygotic), as they share a placenta |
| Monochorionic monoamniotic (MCMA) | <1% | Both babies share a single placenta
Both within the same amniotic sac |
The classification systems are fundamentally similar for triplet pregnancies, but are more complex due to the higher number of possible combinations.
Both twin and triplet pregnancies are classified based on the exact same concepts of chorionicity (the number of outer membranes or placentas) and amnionicity (the number of inner membranes or amniotic sacs).
Information on the classification of triplet pregnancies is omitted as it is very rare and much less likely to be encountered or examined, compared to twin pregnancies.
Risks and Complications
| Type | Risks and complications |
|---|---|
| Dichorionic diamniotic (DCDA) | General risks that apply to all multiple pregnancies:
DCDA twins have the lowest overall risk compared to other types. |
| Monochorionic diamniotic (MCDA) | Carries all the general risks that apply to all multiple pregnancies (see the DCDA row).
PLUS additional risks from a shared placenta:
|
| Monochorionic monoamniotic (MCMA) | Carries all the risks associated with MCDA (see MCDA row).
PLUS additional risks from a shared amniotic sac:
|
The exact same concepts and complications apply to triplet pregnancies, and the risks are significantly amplified. This is again omitted as it is very rare and much less likely to be encountered or examined, compared to twin pregnancies.
Twin-to-Twin Transfusion Syndrome (TTTS)
| Definition | A severe complication specific to monochorionic pregnancies, where babies share a single placenta and blood supply
It occurs when blood flow between the babies becomes unbalanced due to shared blood vessels (anastomoses) on the placenta, as a result:
|
| Risk | TTTS complicates ~15-20% of all monochorionic twin pregnancies:
|
| Maternal symptoms | Often a sudden onset of:
|
| Fetal complications | Consequences of the donor twin (receives too little blood)
Consequences of the recipient twin (receives too much blood)
|
| Diagnosis | TTTS can be detected on ultrasound (see the monitoring and investigation section below for more details)
|
| Management | Primary management (for TTTS diagnosed <26 weeks): fetoscopic laser ablation of the placental blood vessels
Alternatives:
Timing of birth:
|
Surveillance and Testing
Standard antenatal advice applies to all multiple pregnancies (including dietary guidance, lifestyle recommendations, booking bloods, anomaly scanning – see the Antenatal Care article).
However, the maternal and fetal risks are significantly elevated, necessitating the following additional measures.
Identification of Twin / Triplet Pregnancies
Multiple pregnancies are typically detected during the routine dating scan (ultrasound) between 11 and 14 weeks
This scan also helps establish the chorionicity (number of shared placentas) and amnionicity (number of shared amniotic sacs), which define the exact subtype of the multiple pregnancy:
- Lambda sign (or twin peak sign): indicates dichorionic pregnancy (i.e. DCDA if it’s a twin pregnancy)
- T-sign: indicates monochorionic pregnancy (i.e. MCDA or MCMA if it’s a twin pregnancy)
Extended Fetal Ultrasound Surveillance
Extended ultrasounds are necessary in the 2nd and 3rd trimester due to the high risk of fetal growth restriction and to screen for complications like twin-to-twin transfusion syndrome.
| Type of pregnancy | Frequency of scans |
|---|---|
| DCDA twins | Every 4 weeks starting from 24 weeks |
| Monochorionic twins (MCDA or MCMA) | Every 2 weeks starting from 16 weeks |
| ALL triplet pregnancies | Every 2 weeks starting from 20 weeks for uncomplicated trichorionic triplets and 16 weeks for dichorionic or monochorionic triplets |
Specific features evaluated during ultrasound surveillance:
- Estimated fetal weight and discordance
- Screening for twin-to-twin transfusion syndrome
- Amniotic fluid levels in each sac
- Fetal bladders (lack of urine in 1 bladder is a sign)
- Umbilical artery doppler
- Middle cerebral artery peak systolic velocity
Preterm Birth Screening
Routine preterm birth screening is required due to the significantly higher risk of spontaneous preterm birth:
- Offer a single ultrasound scan to measure cervical length between 16-20 weeks
- If cervix is short (≥25 mm) → vaginal progesterone once daily until 34 weeks
Do NOT use fetal fibronectin testing alone or home uterine activity monitoring to predict preterm birth.
Do NOT routinely offer bed rest, oral tocolytics, arabin pessaries, or cervical cerclage to prevent preterm birth.
Management
Planned Delivery Timing
Due to increased risk of fetal death in 3rd trimester, planned early birth is universally recommended, assuming uncomplicated:
| Pregnancy type | Planned birth timing |
|---|---|
| Dichorionic diamniotic (DCDA) twins | 37 weeks |
| Monochorionic diamniotic (MCDA) twins | 36 weeks |
| Monochorionic monoamniotic (MCMA) twins | 32-33+6 weeks |
| Trichorionic or dichorionic triplets | 35 weeks |
Mode of Birth
| Pregnancy type | Mode of birth |
|---|---|
| Dichorionic diamniotic (DCDA) twins | If the pregnancy is over 32 weeks:
|
| Monochorionic diamniotic (MCDA) twins | |
| Monochorionic monoamniotic (MCMA) twins | Planned Caesarean section is routinely offered |
| Triplets |
Intrapartum Care
Key principles include:
- Epidural analgesia is actively recommended for vaginal twin birth (improves success rate of any required assisted vaginal birth and allows rapid emergency Caesarean section if needed)
- Continuous CTG is offered instead of intermittent auscultation
- Bedside ultrasound: at the onset of established labour, a portable ultrasound is used to confirm fetal presentations and locate the hearts
- Active management of 3rd stage of labour with uterotonic drugs (e.g. oxytocin) – due to increased risk of postpartum haemorrhage