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Multiple pregnancy (or multifoetal gestation) refers to pregnancy where there’s more than one foetus. Twin pregnancies account for 1 – 2% of all pregnancies, but triplets and quadruplets are very rare.
Multiple pregnancy is a risk factor for virtually all obstetric complications. As such, they require close monitoring and follow-up.
The incidence of multiple pregnancy has been rising slightly since the 1970s due to increased use of IVF and increasing maternal age.
- In vitro fertilisation
- Increased maternal age
- Family history of twins
Classification and epidemiology
- Monozygotic twins (30% of twins)
- Dichorionic, diamniotic (DCDA)
- Monochorionic, diamniotic (MCDA)
- Monochorionic, monoamniotic (MCMA)
- Conjoined twins (extremely rare)
- Dizygotic twins (70% of twins)
- Dizygotic twins are always DCDA
DCDA twins accounts for 80% of all twin gestations. The foetuses each have their own placenta and amniotic sac. They do not share circulation.
MCDA twins account for 20% of all twin gestations. The foetuses share a placenta but have their own amniotic sacs. All monochorionic twins share a circulation.
MCMA twins are very rare (0,01%). The foetuses share one placenta and one amniotic sac.
MCMA twins have a worse prognosis than MCDA, which have a worse prognosis than DCDA twins.
Monozygotic twins are the result of a mitotic division of a zygote originating from a single ovum fertilised by one sperm. The chorionicity and amnionicity depend on how long after fertilisation this division occurs. If division occurs within the first 3 days, DCDA twins develop.
Dizygotic twins are the result of a double conception originating from two ova fertilised by two sperms.
Multiple pregnancy can be suspected when the size of the uterus is bigger than expected (increased symphysis fundal height, abdominal circumference), or if the mother has severe hyperemesis. It should also be suspected in those who had infertility treatment.
Diagnosis and evaluation
The only reliable way to diagnose multiple pregnancy is by ultrasound, which can also determine the chorionicity and amnionicity. Determining the chorionicity is important as it affects the prognosis and management.
The ideal time to determine chorionicity is in weeks 10 – 14. The so-called lambda sign can be present on ultrasound in dichorionic pregnancies. This sign is the appearance of a potential space between the placentas. In monochorionic pregnancies, the T sign can be seen instead.
As complications are frequent in twin pregnancies, delivery is often by C-section. However, normal labour is allowed if there are no complications, in cases where:
- Both foetuses are in vertex presentation
- The term is > 33 weeks
- There is no preeclampsia, diabetes mellitus, or other complication
- The first foetus is > 1800 g and the second is not more than 500 g more
If there are no indications for C-section, vaginal birth is induced early (weeks 36 – 39, depending on chorionicity).
In case vaginal birth is attempted when the first foetus is in breech and the second in vertex presentation, a serious complication called “locked twins” may develop, where the head of the second foetus slips into the pelvis and prevents the engagement of the head of the first foetus. This has a high mortality and is the reason C-section must always be performed in case of these presentations. (Although Norwegian guidelines state that it’s so rare that it’s not a contraindication for vaginal delivery).
C-section is always indicated for triplets or quadruplets.
- Premature labour
- Intrauterine growth restriction
- Developmental disorders
- Intrauterine demise
- Twin-twin transfusion syndrome
- Placenta praevia
- Postpartum haemorrhage
Intrauterine demise of a single foetus does not harm the other in case of dichorionic pregnancy. However, in case of monochorionic pregnancy, death of one foetus compromises the cotwin’s life too. This is related to foetal exsanguination due to the shared circulation between the two, which causes hypoxic-ischaemic injury of the living twin.
Twin-twin transfusion syndrome (TTTS) is a possible complication of monochorionic pregnancies where there is an unequal blood flow between the twins. One twin “transfuses” its blood flow to the other, causing one the donor twin to be hypovolaemic and the recipient to be hypervolaemic. Its highly lethal and has a high risk of caused disability in survivors. Its severity is classified according to the Quintero classification.
TTTS typically develops between weeks 15 – 25. It’s diagnosed when ultrasound examination reveals one twin with polyhydramnios and the other with oligohydramnios. TTTS may be managed by serial amnioreduction, laser coagulation of the anastomoses with foetoscopy, or as a last line option, selective foeticide.