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Monday, 8 April 2013

Types of Twin Pregnancies

I recently saw a young lady with no history of twins in the family thoroughly shocked but overjoyed (and maybe overwhelmed) when I told her she had twins. She was in her early pregnancy at 8 weeks. An ultrasound scan showed two distinct waterbags and two foetal echoes with heartbeats. This was a non-identical twin pregnancy which is more common than identical twins. It results from the fertilisation of two separate eggs that is released from the ovary. The incidence of twin pregnancies is 1% of which 2/3rds will be non identical and 1/3rd identical.
 Identical twins result from the splitting of the zygote after the egg and sperm have fused. There are two types of identical twins, one of which has two separate placentas and the other type has one single placenta. The types of identical twins is determined by the time at which cleavage of the zygote takes place. If before 4 days, the dichorionic diamniotic DC DA twin with two placentas is formed. This accounts for 25% identical twins. If between 4 and 8 days, the monochorionic diamniotic MC DA twin with a single placenta forms. This is the most common accounting for 75% identical twins. If cleavage takes place between 8 and 12 days, the monochorionic monamniotic MC MA twin with a single placenta occurs. This rare at less than 1%. Cleavge after 12 days may result in conjoined twins.
The importance of knowing which type of twin pregnancy lies in the complication rates. The MC MA twin unfortunately has an 85% chance of mortality mostly due to cord entanglement. The MC DA and DC DA have complications mainly resulting from twin-twin transfusion, and intra-uterine growth retardation resulting from the stronger twin drawing more nutrition and depriving the other. So as an obstetrician it is not always `joyful' to see a twin pregnancy and we have to be extra vigilant in the obstetric care of the patient.

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