Multiple Birth Rates In Twin Pregnancies

Twins are a unique proposition of nature: two offspring produced during a single pregnancy. Depending on their origin, they can either be monozygotic ("identical"), meaning that they can develop from just one zygote that will then split and form two embryos, or dizygotic ("fraternal"), meaning that they can develop from two different eggs, each of which are fertilised by separate sperm cells.

The incidence of multiple births has risen in the past 30 years. In 2009, 16 women per 1000 givingbirth in England and Wales had multiple births compared with 10 per 1000 in 1980. This rising multiple birth rate is due mainly to increasing use of assisted reproduction techniques, including in vitro fertilisation (IVF). Up to 24% of successful IVF procedures

The risk of preterm birth is also considerably higher in multiple pregnancies than in singleton pregnancies, occurring in 50% of twinpregnancies (10% of twin births take place before 32 weeks of gestation). The significantly higher preterm delivery rates in twin pregnancies mean there is increased demand for specialist neonatal resources. Risks to the babies depend partly on the chorionicity and amnionicity of the pregnancy.

Feto-fetal transfusion syndromealso known as twin-to-twin transfusion syndrome, is a condition associated with a shared placenta and accounts for about 20% of stillbirths in multiple pregnancies. Additional risks to the babies include intrauterine growth restriction and congenital abnormalities. In multiple pregnancies, 66% of unexplained stillbirths are associated with a birthweight of less than the tenth centile, compared with 39% for singleton births. Major congenital abnormalities are 4.9% more common in multiple pregnancies than in singleton

If there is only one membrane the pregnancy is described as monochorionic; if there are two, the pregnancy is dichorionic; and if there are three, the pregnancy is trichorionic.

Discordance is defined with the larger twin as the standard of growth and is calculated by the following equation: (larger estimated or actual weight – smaller estimated or actual weight)/ larger estimate or actual weight) Miller J et al, 2012 (2). While acknowledging the lack of consensus on the precise threshold of discordance that is linked with complications, the ACOG practice bulletin on multiple gestation considers a 15-25% difference in actual weight among twins to be discordant (3).

The consensus statement by the Society of Obstetricians Gynecologists of Canada (SOGC) specifies that discordance is a difference of abdominal circumference (AC) of 20 mm or estimated fetal weight (EFW) difference of 20%. The SOGC recommends that the EFW be derived from biparietal diameter with AC or a combination of AC and femur length (4).

In 2006, there were 137,085 twin pairs born in the United States; if 16% were discordant, it is estimated that there are approximately 22,000 discordant twin pairs born per