In humans, a multiple birth is defined as a birth which involves more than one baby. By default, a multiple pregnancy is considered to be high risk, because there are a number of risks of multiple birth which need to be carefully considered by a pregnant mother and her support team. Multiple births, especially high order multiples with large numbers of children, are on the rise, thanks to the use of fertility treatments, which drastically increase the probability of multiple birth.
Twins have been known throughout human history, and although the twinning rate has increased dramatically, twin pregnancies are treated differently than pregnancies involving high order multiples, because they are believed to be less dangerous. However, a mother who is pregnant with twins is still subject to the risks of multiple birth, from gestational diabetes to premature labor, and although twin pregnancies are less risky than high order multiples, they can still be dangerous.
One of the most pressing risks of multiple birth is premature labor. Generally, human gestation lasts for 40 weeks, and babies which are born at 40 weeks tend to be healthier. With twins, gestation may approach 37 or even 35 weeks, and this can be very dangerous. In high order multiples, gestation periods may shrink even more, because the human body is simply not equipped to carry high numbers of babies. Premature infants or preemies have a very low birth weight, and they are at risk of a number of health problems.
For the mother, the risks of multiple birth include a variety of pregnancy-related health conditions like gestational diabetes, kidney infections, breakthrough bleeding, anemia, preeclampsia, and hypemesis, uncontrolled and violent vomiting which can be very unpleasant. Placental problems like placenta previa and abruptio placenta are also more common with multiples, as is a condition known as “incompetent cervix,” in which the cervix naturally opens, allowing the babies to born prematurely or triggering a miscarriage.
The risks of multiple birth also include potential problems for the infant. Multiples experience higher rates of developmental delays, birth defects, and conditions like cerebral palsy. Problems with fetal development are believed to be more common in multiple births because of the increased load on the mother's body.
Some studies have also linked specific health problems with the use of fertility treatments, distinguishing between naturally conceived multiples and those conceived with the use of fertility drugs. More study is needed before establishing a firm link.