Overview of Multiple Pregnancy
What is multiple pregnancy?
Multiple pregnancy is a pregnancy with 2 or more fetuses. Names for these include
While multiples account for only a small percentage of all births (about 3%), the
multiple birth rate is rising. According to the National Center for Health Statistics,
the twin birth rate has risen 70% since 1980. It is currently 32.6 per 1,000 live
births. The birth rate for triplets and other higher-order multiples rose dramatically,
but has slowed since 1998.
What causes multiple pregnancy?
There are many factors related to having a multiple pregnancy. Naturally occurring
factors include the following:
Heredity. A family history of multiple pregnancy increases the chances of having twins.
Older age. Women over 30 have a greater chance of multiple conception. Many women
today are delaying childbearing until later in life. They may have twins as a result.
High parity. Having 1 or more previous pregnancies, especially a multiple pregnancy,
increases the chances of having multiples.
Race. African-American women are more likely to have twins than any other race. Asian
and Native Americans have the lowest twinning rates. White women, especially those
over age 35, have the highest rate of higher-order multiple births (triplets or more).
Other factors that have greatly increased the multiple birth rate in recent years
include reproductive technologies, including the following:
Ovulation-stimulating medicines, like clomiphene citrate and follicle stimulating
hormone (FSH) help produce many eggs. If fertilized, they can result in multiple babies.
Assisted reproductive technologies. In vitro fertilization (IVF) and other methods
may help couples conceive. These technologies often use ovulation-stimulating medicines
to produce multiple eggs. These are then fertilized in the lab and returned to the
uterus to develop.
How does multiple pregnancy happen?
Multiple pregnancy usually happens when more than 1 egg is fertilized and implants
in the uterus. This is called fraternal twinning. It can produce boys, girls, or a
combination of both. Fraternal multiples are simply siblings conceived at the same
time. However, just as siblings often look alike, fraternal multiples may look very
similar. Fraternal multiples each have a separate placenta and amniotic sac.
Sometimes, 1 egg is fertilized and then divides into 2 or more embryos. This is called
identical twinning. It produces all boys, or all girls. Identical multiples are genetically
identical. They usually look so much alike that even parents have a hard time telling
them apart. However, these children have different personalities and are distinct
individuals. Identical multiples may have individual placentas and amniotic sacs.
Most share a placenta with separate sacs. Rarely, identical twins share 1 placenta
and a single amniotic sac.