Multiple Pregnancy is much more common today than they were in the past.

According to the U.S. Department of Health and Human Services, the twin birth rate has increased over 50% since 1980, and triplet, quadruplet, and higher order multiple births have increased at an even higher rate. There are more multiple births today in part because more women are receiving infertility treatment, which carries a risk of multiple pregnancies. Also, more women are waiting until later in life to attempt pregnancy, and older women are more likely than younger women to get pregnant with multiples, especially with fertility treatment. Although major medical advances have improved the outcomes of multiple births, they are still associated with significant medical risks and complications for the mother and children. If you are at risk for a multiple pregnancy, this booklet will help you learn how and why multiple pregnancies occur, and the unique issues associated with carrying and delivering a multiple pregnancy.

Twins Are The Most Common Form of Multiple Pregnancy

You may know someone who has twins, but do you know how twins occur and how they develop?
There are two types of twins: identical and fraternal (non-identical). Identical twins occur when a
single embryo, created by the union of a sperm and egg, divides into two embryos. Each embryo is
monozygotic, genetically identical. When the twinning process occurs early in the pregnancy, in the
first two weeks after conception, each fetus has its own separate gestational sac and placenta.
However, if the twinning process occurs later, after the placenta has formed, the two embryos may be
together in one sac. Non-identical twins occur when two separate eggs are each fertilized by a
separate sperm. The two embryos that result are dizygotic, not genetically identical. Therefore, the
twins are not identical. If ultrasound shows twins together in a single sac, they are identical. Twins that are in the same sac but separated by a thin membrane, the amnion, are also identical. Twins in two different sacs may be identical or non-identical.

The “Vanishing Twin Syndrome”

Sometimes, very early in a twin pregnancy, one of the fetuses “disappears.” This is referred to as
“vanishing twin syndrome.” Even after ultrasound has shown heart movement in twins, spontaneous
loss of one of the fetuses occurs in up to 20% of twin pregnancies. Spontaneous losses are even
higher in triplet and quadruplet pregnancies. A fetal loss rate of 40% may occur in pregnancies withtriplets or more. When a fetus is lost in the first trimester, the remaining fetus or fetuses generally continue to develop normally, although vaginal bleeding may occur.

Ultrasound examinations performed early in the fifth week of pregnancy may occasionally fail to
identify all fetuses. An “appearing twin” may be found after the fifth week in nearly 10% of nonidentical twin or multiple pregnancies and over 80% of cases of identical twins. After six to eight
weeks, ultrasound should provide an accurate assessment for the number of fetuses.

Risk Factors For Multiple Pregnancy

Naturally, twins occur in about one in 50 to one in 100 pregnancies (1 – 2%), triplets in about one in
8,000 pregnancies, and quadruplets in about one in 700,000 pregnancies. The main factor that
increases multiple pregnancy is the use of infertility treatment, but there are other factors.
Your race, age, heredity, or history of prior pregnancy does not increase your chances of having
identical twins, but does increase your chance of having nonidentical twins. Infertility treatment
increases your risk of having twins, both identical and non-identical.

Race. Twins occur in approximately 1 of every 90 pregnancies in North America. The incidence is
higher in Africa, with a rate of 1 in 20 births in Nigeria. Twins are less common in Asia. In Japan, for
example, twins occur only once in every 155 births.

Heredity. The mother’s family history may be more significant than the father’s. Non-identical twin
women give birth to twins at the rate of 1 set per 60 births. However, non-identical males father twins at a rate of 1 set per 125 births.

Maternal age and prior pregnancy history. The frequency of twins increases with maternal age and number of pregnancies. Women between 35 to 40 years of age with four or more children are three times more likely to have twins than a woman under 20 without children.

Maternal height and weight. Non-identical twins are more common in large and tall women than in
small women. This may be related more to nutrition than to body size alone. During World War II, the incidence of non-identical twinning decreased in Europe when food was not readily available.
Fertility Drugs and Assisted Reproductive Technology Multiple pregnancy is more common in women who utilize fertility medications to undergo ovulation induction. Of women who achieve pregnancy with clomiphene citrate, approximately 5% to 10% are twins, and less than 1% are triplets or greater.

Approximately 20% of pregnancies resulting from superovulation treatment using gonadotropins are multiples. While most of these pregnancies are twins, up to 5% are triplets or greater due to the
release of more eggs than expected. In part, ART procedures such as in vitro fertilization (IVF) were largely responsible for the increase in the multiple birth rate. The risk of multiple pregnancy increases as the number of embryos transferred increases but due to advances in embryology laboratory technique, the transfer of a large number of embryos is seldom performed. Today, superovulation accounts for most of the high order multiples.