The diagnosis of a twin pregnancy is often an exciting, if not overwhelming diagnosis which is generally discovered at the first scan in a pregnancy. Twin pregnancies occur in about 30 of every 1000 pregnancies, with 3 of these pregnancies being monozygotic, or identical.

Twin pregnancies can occur spontaneously via a natural conception, or can occur via IVF or other methods of fertility assistance, usually due to 2 embryos being placed in the uterus or two eggs being released from the ovary which are fertilised naturally.

Twin types

The most important factor to diagnose by ultrasound at 12 weeks gestation for twins is their type, determined by whether they are twins which have occurred from 2 eggs fertilising, called Dizygotic (or fraternal) twins, or 1 egg fertilising and then splitting, called monozygotic (or identical) twins.

The other factor which is important is whether the twins have separate amnionic sacs, called Diamnionic, or share an amnionic sac, called Monoamnionic.

Thus, Dizygotic Diamnionic twins - 2 eggs fertilised, resulting in twins with 2 separate placentas and 2 separate amnionic sacs.

Monozygotic Diamnionic Sacs - 1 egg fertilised which has split at 4-8 days after fertilisation resulting in twins sharing one placenta, but in 2 separate amnionic sacs.

Monozygotic Monoamnionic Sacs - 1 egg fertilised which has split after 9 days from fertilisation, resulting in twins sharing a placenta and an amnionic sac.

Pregnancy symptoms and conditions

As there are 2 babies present, general pregnancy symptoms can be a bit worse. Also, the rate of complicating pregnancy conditions is higher.

Morning sickness

Can be worse due to higher levels of the hCG hormone in the first trimester.


Twin pregnancies place greater demands on the body and so tiredness can be more severe than in singleton pregnancies.

Aches and pains

As the uterus size is greater at every stage due to the presence of 2 babies instead of one, aches and pains of the back, hips, pelvis and generally everywhere else are greater than when there is one baby.


In a twin pregnancy, two babies are using up their mother’s iron at a greater rate than one baby, hence iron levels fall quicker in twin pregnancies.

Gestational diabetes

The presence of two placentas in a Dygotic pregnancy, or a larger placenta in a Monozygotic pregnancy increases the risk of gestational diabetes, due to a larger amount of Human placental lactogen being produced from the placenta.

Hypertension and Pre-Eclampsia

This is also more frequent in twin pregnancies.

Premature Birth

Unfortunately, twin pregnancies have a much higher incidence of premature birth.

This can occur due to:

  • Labour starting prematurely.
  • The membranes rupturing prematurely leading to a premature labour or need for an earlier birth due to an infection starting in the uterus.
  • Early delivery required due to issues with the babies’ growth, or development of pre-eclampsia.


As the twin pregnancy is a much higher risk pregnancy, the management is different and closer monitoring is required. The level of monitoring depends on the twin pregnancy type. The usual pregnancy blood tests, Down Syndrome screening and 20-week ultrasound are still performed, but added tests are required.

Dichorionic Diamnionic

Extra growth scans are required at 28 and 34 weeks gestation assuming all is going well. More frequent growth scans may be required. Delivery will be planned for 37-38 weeks gestation, either by induction of labour if the leading twin engages head first in the pelvis, or by Caesarean Section.

Monochorionic Diamnionic

These twins are at risk of twin-to-twin transfusion syndrome (TTTS), a condition when there is a connection between the blood circulations of both twins, and one baby takes blood from the other. This condition can endanger the lives of both twins.

As a result, an ultrasound scan of both twins is performed every 2 weeks from 16 weeks. If there is no evidence of TTTS on the scans, and the pregnancy progresses well, delivery will be the same as for DIZYGOTIC twins at around 37-38 weeks either by induction of labour or by Caesarean Section.

Monozygotic Monoamnionic Twins

These twins are particularly high risk as they share a placenta, so are at risk of TTTS, but also as they share a sac, their cords can become entangled, leading to oxygen deprivation in the uterus causing cerebral palsy or even death to both twins. This twin type is managed in a tertiary centre like The Royal Brisbane Hospital and these babies are often delivered early at around 32 weeks due to high-risk nature of the pregnancy.

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