Multiple pregnancies (usually twins or triplets) occur in 2-3% of all gestations. The incidence of multiple pregnancies is increasing, principally due to the increasing age of mothers and treatments to assist women conceiving (eg ovulation induction medication or IVF technologies).
The chance of problems occurring in a pregnancy is increased when there is more than one baby – particularly preterm birth and fetal growth problems. Due to the increased chance of a pregnancy complication, your doctor will arrange for you to have more ultrasound reviews than would normally occur in a singleton pregnancy. Ultrasound can provide you and your doctor with reassurance that your babies are growing well, or on occasions may identify some of the problems that require further treatments.
What is zygosity?
Twins (2 babies in the uterus) are the most common type of multiple pregnancy, accounting for >90% of multiple births. There are two genetic types of twins (medically termed zygosity):
Dizygotic twin pregnancy – where two eggs are fertilised in the same cycle (65% of all twins). These are sometimes called fraternal twins as they have the same genetic makeup as do brothers and/or sisters.
Monozygotic twin pregnancy – where a single egg is fertilised but later divides to form two fetuses (35% of all twins). These are sometimes called identical twins as they have virtually the same genetic makeup as each other.
Dizygotic twins tend to occur more frequently with increasing maternal age, couples with a family history of twins, particular racial types and from assisted reproductive technology (eg IVF, ovulation induction).
Monozygotic twins tend to occur with equal frequency across maternal age and race. They may also be associated with ovulation induction and blastocyst transfer techniques.
What is chorionicity?
Chorionicity refers to the type of placentation in twins. It is the critical information necessary for the antenatal management of twin pregnancies.
There are two types of placentation in twins:
Dichorionic placentation (80% of twins) Dichorionic twins have two separate placentae that may be widely separated in the uterus or situated close together depending on where the two embryos implanted in the uterus. Each baby is independent of the other. Dizygotic twins always have dichorionic placentation.
Monochorionic placentation (20% of twins) Monochorionic twins have a single placenta with each fetus sharing the placenta with the co-twin. This indicates that both fetuses developed from the single fertilized egg.
What is amnionicity?
This refers to the number of fluid filled sacs in which the fetuses reside.
All dichorionic twins have two separate amniotic sacs. Hence this type of twinning is termed as dichorionic diamniotic twins.
Monochorionic twins can have two separate sacs or a single sac depending on the time of division of the fertilized egg.
What are the major risks with twin pregnancies?
The major complications of twin pregnancies include:
Preterm birth (>50%)
Intrauterine growth restriction
Discordant growth profiles (where one twin is significantly larger than its co-twin)
Discordant amniotic fluid volumes between the two fetuses
Placenta previa (part or all of the placenta covers part or all of the lower uterine segment and cervix)
Fetal structural anomalies
Complications specific to monochorionic placentation (Twin-Twin Transfusion Syndrome, discordant structural anomalies, severe growth discrepancies)
As the complication rate for monochorionic twins is far greater than that for dichorionic twins your doctor will arrange for more frequent ultrasound surveillance in monochorionic twin pregnancies.