Multiple pregnancy, usually referred to as multiple gestation, is one in
which more than one fetus develops simultaneously in the mother's
The frequency of multiple births in the United States has been steadily
increasing with advances in reproductive technologies. It is estimated
that pregnancies resulting from assisted technologies have a 25–30
percent incidence of
and a 5 percent incidence of triplets. The frequency of naturally
occurring twins is approximately one in 80 births; however the frequency
of multiple births in the United States for 2002 was as follows:
Twin birth is by far the most common multiple birth. There are two types
of twin pregnancy: fraternal and identical. Fraternal twins develop from
two separate ova released at the same time and fertilized by two separate
sperm. Fraternal twins are referred to as dizygotic twins, meaning that
two unions of two gametes or male/female sex cells occurred to produce two
separate embryos. Characteristically, with fraternal twins, each has its
own placenta and amniotic sac. They may be the same or different sex,
occur twice as frequently as identical twins, and have a mortality rate of
11.5 percent. Identical twins represent the splitting of a single
fertilized zygote (union of two gametes or male/female sex cells to
produce a developing embryo) into two separate individuals. Identical
twins will have the same DNA, genetic material (genotype), but it may be
expressed differently (phenotype). There are three ways identical twins
can exist in the uterus: dichorionic-diamniotic twins;
monochorionic-diamniotic twins; monochorionic-monoamniotic twins. In the
instance of dichorionic-diamniotic twins, division of the fertilized egg
occurs within 72 hours past fertilization, before the inner cell mass has
developed. About 30 percent of identical twins have this classification,
and each twin has its own chorion, amnion, and placenta. The mortality
rate for this type of twinning is 9 percent. With monochorionic-diamniotic
twins, division occurs in the range of four to eight days after
fertilization, and the inner cell mass divides in two. The placenta has
one chorion and two amnions, so each twin has its own amniotic sac.
Approximately 68 percent of identical twins are in this classification,
and they have a mortality rate of 25 percent. Thirdly,
monochorionic-monoamniotic twins are contained in the same amniotic sac.
The division of the fertilized egg in this case occurs nine to 13 days
past fertilization or near the time of implantation in the uterus. Since
they share an amniotic sac, they have an increased risk of their umbilical
cords becoming entangled or knotted. Only 2 percent of identical twins are
in this classification, and they have a mortality rate of greater than 50
percent. If a complete separation does not take place during the division
process, the result is Siamese (or conjoined) twins.
The human female typically releases only one egg every menstrual cycle. A
hormone called progesterone, released by the first egg to be produced,
prevents any other egg from maturing during that cycle. When this control
fails, fertilization of more than one egg is possible. Fertility drugs
inhibit these controls, allowing multiple gestation to occur. It seems as
if, however, that more pregnancies start out naturally with twins than was
originally believed. The development of improved technology, such as
ultrasound, has made it possible to determine more accurately the early
pregnancy loss rate of
twins to include both complete pregnancy loss and spontaneous resorption
of one twin, frequently referred to as the vanishing twin phenomenon.
Recent research suggests that 75 percent of twin pregnancies are lost
before the end of the first trimester. Moreover, only about 50 percent of
pregnancies diagnosed in the first trimester with twins result in the
birth of two live infants.
An old adage related to multiple gestation is the human female was not
meant to have more than twins because she only had two breasts for
feeding. Of course, pregnancies with more than two babies have occurred
throughout history. However, once the number of babies reaches three,
overexpansion of a woman's uterus begins to cause difficulties. The
implantation of several embryos and placentas in the endometrium of the
uterus results in a competition for space and inevitably some implant in
an area without good circulation. During a pregnancy, it is essential that
the uterus be well perfused to sustain the fetus with nutrients and
oxygen. A lack of oxygen can cause central nervous system damage in the
fetuses that implanted in a less than desirable area. Since the human
female was not made to carry an indefinite number of fetuses, multiple
gestations can have many of the following complications:
Causes and symptoms
Twinning seems to run in some families, is mainly confined to fraternal
(dizygotic twins) and seems to be entirely a property of the mother, not
the father. The primary cause is an increased chance of multiple
ovulation, when a woman releases two or more eggs. Another major factor is
maternal age; a woman who gives birth at 37 is four times more likely to
have fraternal twins than at age 18. The 37-year-old is also more likely
to be unable to conceive, since many women's ovaries are already
starting to fail at that age. The third major factor is race; West
Africans are ten times more likely to have fraternal twins than Chinese or
Japanese, with Caucasians intermediate. This increased chance is also seen
in African Americans. In addition, the more pregnancies a woman has had,
the greater her chances of having twins. In fact, by the fourth or fifth
pregnancy, the likelihood of having twins is four times higher than it was
for the first pregnancy.
The use of assisted reproduction techniques, particularly ovarian
stimulation, has caused a dramatic increase in the number of twin and
higher multiple births. The normal process of single ovulation is
interrupted because fertility drugs permit more than one egg at a time to
mature and be released. The first drug to be used for this was clomiphene
(Clomid). This was followed by the development of two natural hormones,
follicle-stimulating hormone and chorionic gonadotrophin (Pergonal) to
produce multiple eggs ovulation. The chance of multiple gestation with in
vitro fertilization (IVF) is about the same as with the use of fertility
drugs, because several embryos are inserted into the womb to increase the
odds of conception. Similarly, other fertility techniques such as gamete
intrafallopian transfer (GIFT) and zygote intrafallopian transfer (ZIFT)
are also more likely to result in multiple gestations. The use of
intrauterine insemination or artificial insemination (the injection of
sperm into a woman's uterus with a syringe) is the only fertility
treatment that does not increase the chances of conceiving
multiples—of course, the woman is usually taking fertility drugs
with this procedure also.
If a multiple pregnancy occurred spontaneously, the obstetrician would
suspect a problem with the dates because the uterus would grow faster than
usual. The gestational age of a pregnancy is determined from the first day
of the last menstrual period (LMP). In a multiple gestation, the uterine
measurements would be larger than dates, which normally correspond. If
multiple gestation is suspected, an ultrasound may be performed to
determine the gestational age of the fetus or to check for more than one
fetus. With the use of assisted reproductive technology, an ultrasound is
usually performed with ten days to see if any of the embryos were
successful with implantation, and a multiple gestation would be revealed
at that time. Following the birth of multiples, the placenta is carefully
examined to determine if they are fraternal or identical. One placenta
indicates identical twins.
A multiple pregnancy almost always means increased monitoring and
surveillance for complications. This often means more frequent visits to
the healthcare provider, serial ultrasounds to make sure that the babies
are growing satisfactorily,
to check for lung development, and close monitoring for preterm labor.
Ultrasound examinations play an important role in the care and treatment
of multiple gestations. It assists with dating the pregnancy, determining
the number of fetuses, detecting fetal anomalies, following the growth and
development of each fetus, and serves to monitor the length of the cervix
in anticipation of preterm labor. Premature birth is the constant threat
of multiple gestation, and the primary threat of
is related to lung development. Premature infants lack a substance,
called surfactant, that permits their lungs to expand and breathe
normally. If it becomes apparent that a multiple gestation is going to
have a preterm delivery, the mother will be given an injection of a
steroid, beta-methasone, to help the lungs mature. It is more beneficial
if the steroid can be given twice in a 48 hour period; however, if there
is not time for this, surfactant has been developed that can be
administered into the lungs of a premature infant to facilitate breathing.
An additional problem with the premature infant involves the lack of body
fat. In a normal pregnancy the fetus spends the last four to six weeks
growing and gaining weight, primarily body fat. This fat helps a newborn
maintain his or her body temperature. Since premature infants do not have
this fat, they use energy they cannot afford to stay warm.
Although research has shown that bed rest is not effective, a woman is
often placed on bed rest during a multiple pregnancy to try to prevent
pre-term labor and delivery. If preterm labor is impossible to control at
home, the mother may be hospitalized and medication used to attempt to
control contractions and dilatation of the cervix. Multiple gestations
greater than twins in number are almost always delivered via cesarean
There are no specific treatments to alleviate medical difficulties caused
by multiple pregnancies; however, there are supportive measures that may
help both mother and children recover from the birthing process. There are
treatments to encourage breast milk production and to combat postpartum
difficulties. Various homeopathic remedies and massage can be helpful to
both mother and children during the early adjustment period after birth.
—Thin, tough, innermost layer of the amniotic sac.
—The thin tissue that creates the walls of the amniotic sac.
—The outer membrane of the amniotic sac. Chorionic villi develop
from its outer surface early in pregnancy. The villi establish a
physical connection with the wall of the uterus and eventually develop
into the placenta.
—A building block of inheritance, which contains the instructions
for the production of a particular protein, and is made up of a
molecular sequence found on a section of DNA. Each gene is found on a
precise location on a chromosome.
—The period from conception to birth, during which the developing
fetus is carried in the uterus.
—The plural of ovum, it is the female reproductive cell.
—To release a mature egg for fertilization.
—The organ that provides oxygen and nutrition from the mother to
the unborn baby during pregnancy. The placenta is attached to the wall
of the uterus and leads to the unborn baby via the umbilical cord.
—Plural, sequalae. An abnormal condition resulting from a
previous disease or disorder.
—The result of the sperm successfully fertilizing the ovum. The
zygote is a single cell that contains the genetic material of both the
mother and the father.
Mothers with multiple pregnancy should be especially careful to get
adequate prenatal care, including any necessary
or recommended tests. Because
of the extra stress on the mother's body, increased rate of
complications, and threat of prematurity, the mother should be vigilant in
making sure she gets enough rest, reduces stress, and maintains a healthy
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Association of Women's Health, Obstetric and Neonatal
2000 L Street, N.W. Suite 740, Washington, DC 20036. Web site:
American College of Obstetricians and Gynecologists.
409 12th Street, S.W., P. O. Box 96920, Washington, DC 20090.
Questions about Multiple Pregnancy.
[cited March 6, 2005]. Available online at: