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?
During pregnancy we often cannot determine the zygosity of twins (unless one is a boy and the other a girl). You will hear the term chorionicity mentioned rather than zygosity. Chorionicity refers to the type of placentation in a twin pregnancy and is used to stratify obstetric risk level.
There are two types of placentation in twin pregnancies:
- Dichorionic placentation (most frequent: 80% of all twins)
- Monochorionic placentation (20% of twins)
Dichorionic twin placenta demonstrating
the 'Twin Peak' sign
A dichorionic twin placenta means that there are two separate placental discs – they may be widely separated in the uterus or in close proximity depending upon where the two embryos implanted in the uterus. All dichorionic twins have two separate sacs (medically termed amniotic sacs) in which the babies live. Thus this sort of twinning is described as dichorionic and diamniotic (two placentas and two fluid-filled sacs, each containing one baby). Apart from being housed in the same uterus in this form of twinning each baby is independent of the other. Dizygotic twins always have this form of placentation. Monozygotic twins may have a dichorionic placenta if the single fertilised egg divides in the first 3 days after conception.
Early monochorionic twin placentation
A monochorionic twin pregnancy means that there is one single placental disc with each baby sharing the placental mass with the co-twin. Inextricably dependent upon each other, this form of placentation can be viewed as conjoined twinning of the placenta. This form of twinning means that both fetuses developed from the same fertilised egg, with division occurring more than 3 days after fertilisation.
The amnionicity (number of fluid filled sacs) in monochorionic twinning is related to the time of division of the single fertilised egg.
- Monochoronic diamniotic twins: When the egg divides 3-8 days after fertilisation two separate amnions are present. This is the most common type of monochorionic placentation.
- Monochorionic monoamniotic twins: If the egg divides between day 8 and 13 after fertilisation, a single placenta and a common amniotic sac occurs. Only 1% of monochorionic twin pregnancies will have a single sac: it is a very high risk pregnancy with a risk of entanglement of the umbilical cords of the two fetuses.
- Conjoined twins: If the egg divides very late, after two weeks following initial fertilisation, the fetuses are fused within a single pregnancy sac. Fortunately this is extremely rare as the complication rate for the mother and her conjoined fetuses is extreme.
The chorionicity of a twin or higher order multiple pregnancy can be accurately determined at the time of First Trimester Screening or earlier if an ultrasound scan has been performed in the early first trimester. Amnionicity (or ascertainment of the number of sacs) is only accurately assessable after 8 weeks gestation.
What are the major problems with multiple 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.