Twin pregnancy

Twin pregnancies are the most common multifetal pregnancies

Multifetal pregnancies account for ~ 1% of all pregnancies, but are seen in much higher numbers in populations where in vitro fertilisation (IVF) is a common practice, most of which are twin pregnancies.

A twin pregnancy can be broadly categorised into 

Dizygotic twins

Also known as fraternal twins resulting from independent fertilization of two ova, and always result in dichorionic - diamniotic pregnancies. This feature alone cannot be used to distinguish dizygotic from monozygotic pregnancies as approximately 20% of monozygotic pregnancies will also be dichorionic-diamniotic (resulting in ~80% of all twin pregnancies being dichorionic-diamniotic). The most reliable sign of dizygosity on ultrasound is documentation of different sex fetuses.

Risk factors for dizygotic pregnancy:

  • in vitro fertilisation (IVF): particularly use of ovulation induction agents
  • advanced maternal age
  • advanced parity
  • maternal family history
  • ethnicity (e.g. Nigerian 1 in 25 in Nigeria to 1 in 155 in Japan) 4 
Monozygotic twins

Also known as identical twins are due to fertilisation of a single ovum that then separates into two. Unlike dizygotic pregnancy, monozygotic pregnancies do not have maternal age, family history or ethnic predisposition. The time at which this separation occurs determines the chorionicity and amnionicity of the pregnancy.

Division at:

All twin pregnancies have a higher fetal and neonatal mortality with overall rates of fetal mortality being 3-6 times that of a singleton pregnancy and neonatal mortality 7 times that of single pregnancies. This is primarily due to much higher incidence of premature labour and all the sequel thereof. 

The risk of complications and therefore the prognosis is worst for mono-mono and best for di-di:

  • di-di: ~10% perinatal mortality
  • mono-di: ~20% perinatal mortality
  • mono-mono: often >50% perinatal mortality
Monochorionic complications

Monochorionic twin pregnancies share the one placenta and are therefore prone to haemodynamic complications such as:

Amniotic fluid discordance is the most reliable predictor of poorer outcome.

Monoamniotic complications

Monoamniotic twin pregnancies share the one amniotic sac and are prone to entangled cords, as well as all the monochorionic complications (since you cannot be dichorionic-monoamniotic).

Ultrasound assessment of multi-fetal pregnancies

The role of ultrasound is crucial in adequate monitoring of the pregnancy and planing for delivery. The number of fetuses and their chorionicity and amnionicity must be determined, as well as monitoring for complications and anomalies.

Determining chorionicity and amnionicity

It is easiest to determine chorionicity and amnionicity in the first trimester.

Dichorionic

  • separate gestational sacs with chorion extending between them ( < 10 weeks)
  • twin-peak sign or (lambda sign) ( > 10 weeks): thick intertwin membrane, with more than two layers
  • separate placental masses (note however, that most dichorionic pregnancies have placental masses that abut each other
    • however the thickness is however subjective and becomes progressively more difficult to asses (98% accuracy in the first trimester vs 83% later)
  • twins may be of different genders

Monochorionic diamniotic

Monoamniotic

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Article information

rID: 1696
System: Obstetrics
Section: Gamuts
Synonyms or Alternate Spellings:
  • Twin pregnancies

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    Case 1: DCDA twin pregnancy
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