Multiple Births

“Multiple births” refers to more than one foetus developing in the uterus during pregnancy. This can be two foetuses (twins), three foetuses (triplets), or more which is very rare. This section will deal with the different types of twinning that can occur. Twins present more clinical complications than single births as there are two (or more) babies developing in the same confined space and are both feeding from the mother, and they often compete for resources which can result in one foetus becoming dominant and the other becoming nutritionally deprived.

Dizygotic twins

Dizygotic twins are non-identical twins which develop from two separate oocytes which have both been fertilised by different sperm. This happens because two oocytes are released by the mother simultaneously, and once both are fertilised they both implant into the uterine wall. This means usually each foetus develops its own placenta, amniotic sac and chorion, unless the blastocysts implant near each other in which case they share a placenta and chorion. This type of twinning occurs more commonly as age of the mother progress, with incidence doubling at the age of 35.

Dizygotic twinning

A = Dizygotic twins implanting apart from each other, B = Dizygotic twins implanting close to each other

Monozygotic twins

Monozygotic twins are identical twins which develop from a single oocyte which has been fertilised by a single sperm. There are four types, depending on when the zygote cleaves into two:

  • Monozygotic twinsDiamniotic dichorionic: This occurs when the morula divides into two separate entities before the end of Day 3 after fertilisation, before implantation has taken place. Much like dizygotic twins, the twins share nothing and each has their own placenta, amniotic sac and chorion.

  • Diamniotic monochorioinic: This occurs when the blastocyst divides into two separate entities between Days 4 – 8 after fertilisation, before implantation has taken place. These twins share a placenta and chorion, but each has their own amniotic sac.

  • Monoamniotic monochorionic: This occurs when the implanted blastocyst divides into two separate entities between Days 8 – 13 after fertilisation, after implantation has occurred. These twins share a placenta, chorion, and amniotic sac.
    The development of different types of monozygotic twins
    This is very rare (only 1% of monozygotic twins) and dangerous due to the very limited space to develop in, with many possible clinical complications meaning that only half of these twins survive until birth.

  • Conjoined: This occurs when the developing blastocyst divides into two separate entities after Day 14 after fertilisation, and the embryonic disc has begun to form. As the basis of the embryo has formed, cleavage isn’t fully completed and results in two separate entities which are joined together at some point along the body. Conjoined twins are the rarest type and have the lowest survival rate due to sharing organs or complications during partuition.
Conjoined twins
Conjoined twins