A multiple pregnancy is a pregnancy in which more than one fetus develops in the uterus at the same time. Multiple pregnancies occur in 1-2% of pregnancies. The rate of twinning (the bearing of twins) is believed to be underestimated, as twin pregnancies with a singleton (an offspring born singly) birth are usually not recorded as twins.
A multiple pregnancy may be the result of the natural process of twinning, or it may be the result of the woman having taken fertility drugs. Because of the increase in artificial reproductive technology (ART), the incidence of multiple pregnancies has increased. An April 1999 National Vital Statistics report from the Centers for Disease Control and Prevention (CDC) states that since 1980 the number of twins has risen by 52% and the number of triplets and high order multiples (more than three) has increased by 404%. An older maternal age and the use of fertility techniques are seen as the two major factors in these increases. While singletons have a 10% risk of being born preterm, multiple births have a 57% chance of being born prematurely. Premature birth places a neonate at higher risk for morbidity and mortality.
There are two categories of twins: monozygotic and dizygotic. Monozygotic twins are twins that have developed from a single fertilized ovum that split during embryonic development. These twins have the same genetic makeup and are always the same sex. They may be surrounded by one chorion (the outer embryonic membrane of the developing fetus), or may each have their own chorion. They may be surrounded by one amniotic sac (innermost of the membranes surrounding the embryo) or may each have their own amniotic sac. They may share a placenta or may each have their own placenta. These different possibilities depend on the time of the embryonic development at which the division took place. About two to 5% of monozygotic twins will share one amniotic sac. This rare occurrence puts the twins at risk for umbilical cord entanglement, cessation of blood flow, and death.
Double survival of monoamniotic twins is rare. Monozygotic twins may be referred to as identical. Dizygotic twins have developed from two fertilized ova. Their genetic makeup is different, and they are no more similar as any two siblings in a family. They may be the same or different sex. Each have their own chorion, amniotic sac, and placenta. While each twin has its own placenta, the placental implantations may be close enough that they fuse into one. Dizygotic twins may be referred to as fraternal. Multiple pregnancies of three or more fetuses may be the result of a single fertilized egg that splits, of multiple egg fertilizations, or a combination of the two processes.
Twins may not grow at the same rate. When there is 25% or more disparity between them, this is referred to as discordance, which occurs in about 10% of twin pregnancies. An extreme case of discordance occurs in the condition called twin-to-twin transfusion, also known as twin oligohydramnios polyhydramnios sequence. In this situation, one twin becomes the donor twin (receives too little blood from vessels in the fetuses' shared placenta that connect their blood circulations) and the other twin is the recipient (receives too much blood). The donor twin becomes small, pale, hypotensive, and anemic, with very little amniotic fluid. The recipient twin is large, polycythemic, hypertensive, with an excess of amniotic fluid. Both are at risk for heart failure and death.
At the time of delivery twins may be in any of the following combinations: vertex-vertex, breech-vertex, vertex-breech, breech-breech, vertex-transverse, or breech-transverse.
Causes and symptoms
In a woman's menstrual cycle, one egg, or ovum, is released every month. If more than one egg is released, it is possible for each egg to be fertilized separately by different spermatozoans. Fertility drugs encourage the release of more than one egg during the monthly menstrual cycle. In the case of monozygotic twins, only one egg was released and fertilized; but after fertilization it split, and separate fetuses developed. If the split is not complete, conjoined twins develop. Conjoined twins share certain body parts and organs. They may be referred to as Siamese twins. The chance of multiple pregnancy increases with an increase in parity and in maternal age up to about 35 years old, and then the incidence begins to decline. Genetics and racial background also play a role.
A multiple pregnancy is suspected if the woman's uterus is growing too quickly for the gestational age, with excessive maternal weight gain, elevated levels of alpha-fetoprotein (a fetal protein that increases in the mother's blood during pregnancy) levels, unexplained severe maternal anemia, or with the auscultation (listening to sound to aid in diagnosis and treatment) of more than one fetal heartbeat. If undiagnosed at the time of quickening, the mother may feel movement in different parts of the uterus at the same time. Ultrasound can confirm or deny the presence of a multiple pregnancy. Once the multiple pregnancy is confirmed, ultrasonography may be used to check fetal growth over time, and the presence of any anomalies. There is a condition referred to as vanishing twin that occurs in up to 50% of twin pregnancies diagnosed very early by ultrasound. While twin sacs were seen on early sonography, a singleton is born. In these cases, there may have been early pregnancy vaginal bleeding and a lower human chorionic gonadotropin (hCG; a type of hormone) level than would be expected. The placenta often shows a whitish area and the remnant of a gestational sac. The mother and surviving twin (born singly) are both healthy.
The diagnosis of a multiple pregnancy will result in it being treated as a high-risk pregnancy because of associated maternal and fetal risks. In a triplet pregnancy the mother may be offered the choice of selective reduction to twins. However, the literature is unclear as to the overall value of reduction from three to two fetuses. In high order multiples, to decrease the risk of very early preterm birth and potential loss of fetal viability, selective reduction may take place. In selective reduction high order multiples are reduced to triplets or twins. The procedure is usually completed prior to the end of the third month of gestation and involves a chemical injection into one or more developing embryos. A fetus that shows chromosomal damage is usually targeted first. While this process increases the chances of the viability of the remaining fetuses, it carries a significant emotional burden for the mother and partner. It also raises ethical issues concerning the "right-to-life" of a fetus. Efforts are being made in the field of ART to prevent the development of high order multiples in order to avoid this particular situation.
Prognosis for a multiple pregnancy depends on many factors. The higher the number of fetuses, the greater the risks. A twin pregnancy carries significantly more risks than a singleton pregnancy. The risks for triplets are similar to that of twins. The risks increase significantly with multiples of four or higher. Twins have a ten-fold risk of perinatal mortality over singletons.
While many multiple pregnancies have an excellent outcome, it is still considered a high-risk pregnancy. The average gestation for a singleton is 38 to 42 weeks. For twins gestation averages 37 weeks; for triplets, 33 weeks; and for quadruplets, 31 weeks. The mother carrying a multiple pregnancy has an increased risk of:
- premature birth
- pregnancy-related hypertension and preeclampsia
- hydramnios (excess amniotic fluid)
- placenta previa (placenta covering the mouth of the womb-cervix)
- folic acid and iron deficiency
- gestational diabetes
- urinary tract infection
- placental abruption after the vaginal delivery of the first twin (separation of the placenta from the uterus before the baby is born)
- uterine atony (failure of the uterus to contract after birth) and postpartal hemorrhage due to exaggerated stretching of the uterus
- fatigue and backache
- cesarian delivery
The risks to fetuses in a multiple pregnancy are greater than that for a singleton and include:
- premature birth (Preterm labor for twins is seven to ten times more likely than for singletons and is a significant factor in perinatal morbidity and mortality.)
- intrauterine growth restriction
- congenital anomalies
- cerebral palsy with increased risk often due to preterm delivery
- discordance; more common with triplets than with twins
- dead fetus syndrome
- combined pregnancy, in which one twin develops in the uterus while the other is ectopic (other than in the uterus, such as the fallopian tube or peritoneal cavity)
- delayed delivery of second twin
- placental abruption
Health care team roles
While a mother carrying a singleton may have one ultrasound done during the pregnancy, the mother of a multiple pregnancy is much more likely to have several ultrasounds done. The experience, skill, and ability of the ultrasound technician to provide a calm environment can be a great help to the mother and her partner. The nurse working in a high-risk obstetric practice can provide a great deal of teaching both to inform the mother about what to expect and to decrease anxiety through knowledge.
Breech— The buttocks or hind end of the body.
Chorion— The outer embryonic membrane of the developing fetus that gives rise to the placenta. Inside the chorion is the amniotic sac or sacs, inside of which are the fetuses.
Morbidity— Morbidity refers to an illness or disease condition. In statistics it refers to the rate at which a disease occurs.
Mortalit— Mortality means death. In statistics it refers to the rate at which death occurs in a population for a particular disease condition.
Parity— The number of pregnancies with a fetus reaching viable gestation.
Singleton— A singleton is a fetus that develops alone in the uterus.
Transverse— At right angles to the anterior-posterior body axis.
Vertex— The top of the head or highest point of the skull.
Twinning is a naturally occurring phenomenon and cannot be completely prevented. It occurs more often in older mothers. Multiple births due to ART are a concern because a multiple pregnancy represents a complication of pregnancy. Efforts within the ART community are being made to minimize the incidence of high order multiples. Efforts to prevent or minimize maternal and fetal complications will result in closer monitoring. More frequent ultrasounds, biophysical profile, and/or nonstress tests may be ordered. Cervical length and change may be monitored as an indicator of preterm delivery. If both twins are vertex and vaginal delivery is attempted, both fetal heart rates will be monitored. Caesarian deliveries of twins are more common than for singletons. This is especially true in high order multiples. The overall cesarian delivery rate tends to be about 75%.
Creasy, Robert K. and Robert Resnik. Maternal-Fetal Medicine, 4th ed. Philadelphia, PA: W.B. Saunders Company, 1999.
Pillitteri, Adele. Maternal & Child Health Nursing: Care of the Childbearing and Childrearing Family. Philadelphia, PA: Lippincott, 1999.
Scott, James. Danforth's Obstetrics and Gynecology, 8th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 1999.
The National Organization of Mothers of Twins Clubs, Inc. (NOMOTC). Executive Office, P.O. Box 438, Thompson Station, TN 37179-0438. (615) 595-0936. (877) 540-2200. 〈http://www.nomotc.org/〉.