Multiple Pregnancies

WHEC Practice Bulletin and Clinical Management Guidelines for healthcare providers. Educational grant provided by Women's Health and Education Center (WHEC)

Multiple pregnancies are fascinating and challenging situations. 1.05 to 1.35% of pregnancies are twins and the frequency of the twinning process varies widely in different populations. Multiple births are more common nowadays, owing to over-stimulation of ovulation that occurs when ovulation stimulation is done in cases of women with infertility because of ovulatory failure. Moreover, although the dizygotic twinning rate varies widely under different circumstances, the monozygotic twinning rate is "remarkably constant", usually between 3.5 to 4 per 1,000. The frequencies of triplets, quadruplets and so on can be calculated roughly by employing Hellin's hypothesis. It stated that when the frequency of twinning is n, that of triplets is n, of quadruplets n, and so on. The highest number so far recorded is nine offspring.

Type of Twins:

Twins who possess characteristics that make them virtually indisguishable are referred to as "identical", whereas others who very unlike are considered "fraternal". The former always have the same gender, but the later may be different gender. The terms identical and fraternal, although popular, are scientifically less useful and are best replaced by the terms monozygotic, and dizygotic, to indicate the mechanism of origin of the two types of twins.

Monozygotic (identical) Twins:

Because they result from the fertilization of one ovum, monozygotic twins are of the same sex, genetically identical, and very similar in physical appearance. Physical differences between identical twins are caused by environmental factors, e.g., anastomosis of placental vessels resulting in differences in blood supply from the placenta.

Monozygotic twinning usually begins around the end of the first week and results from division of the inner cell mass into two embryonic primordial. Subsequently, two embryos, each in its own amniotic sac, develop within one chorionic sac. Early division of embryonic cells (2 to 3 days) results in monozygotic twins who have two amnions, two chorions, and two placentas that may or may not be fused. About 30% of monozygotic twins result from early division. Late division of embryonic cells (9 to 15 days) results in monozygotic twins in one amniotic sac and one chorionic sac. Such twins are rarely delivered alive. It has been estimated that the frequency of monoamniotic twins among monozygotic twins is about 4%.

Dizygotic (fraternal) Twins:

Because they result from the fertilization of two ova by two different sperms, dizygotic twins may be of same sex or of different sexes. For the same reason, they are no more alike genetically than brothers or sisters born at different times. Dizygotic twins always have two amnions and two chorions, but the chorions and placentas may be fused.

Conjoined Twins:

If the inner cell mass, or the embryonic disc, does not divide completely, various types of conjoined twins may form. These are named according to the regions that are attached, e.g., "thoracopagus" indicates that there is anterior union of the thoracic regions. It has been estimated that about once in every 40 monozygotic twin pregnancies, the twinning is incomplete and conjoined (Siamese) twins result. In some cases, the twins are connected to each other by skin only or by cutaneous and other tissues, e.g., liver. Some conjoined twins can be successfully separated by surgical procedures.

Other Multiple Births:

Triplets occur once in about 8,100 pregnancies and may be derived from (1) one zygote and be identical; (2) two zygotes and consist of identical twins and a single infant or; (3) three zygotes and be of the same sex or of different sexes. Similar combination occurs in quadruplets, quintuplets, sextuplets, and septuplets.

Mortality and Morbidity:

Multiple pregnancies remain a high- risk situation, with published perinatal mortality rates in developed countries ranging between 47 and 120 per 1,000 births for twins and between 93 and 203 per 1,000 births for triplets. The increased risk of death in twin's pregnancy is about five-fold higher, the risk being slightly higher for the second twin. The increased risk of death in twins persists during the first year of life, and it is not until the second year that mortality rates for twins are the same as for singletons.

Approximately 10% of preterm deliveries are twin gestations, and they account for 25% of the perinatal deaths in preterm deliveries. The large majority of deaths in preterm multiple deliveries occur with gestations less than 32 weeks and birth weights under 1500 gm. Intrauterine growth restriction is also common in multiple gestation and makes an important contribution to the high incidence of low-birth- weight and still born babies. It is also associated with significant morbidity. The majority of infants born of a multiple gestation will be of low birth weight, less than 2500 gm and are at risk of significant short-term and long- term handicaps.

Maternal Mortality and Morbidity: They contribute to the high incidence of physical discomfort, prolonged hospitalization, surgical delivery, and social inconvenience, associated with multiple pregnancies. The lay public, who often simply view the pregnancy as twice as interesting and exciting, with twice the expected happy result, generally underestimates the high risks of multiple pregnancies.

Antenatal Risk Factors in Multiple Gestations:

The antenatal risks clearly increase enormously with triplets, quadruplets, and so on. The common problems faced are;

  • Miscarriages: inevitable abortion is at least twice as common in multiple pregnancies as in singleton pregnancy, and a continuing pregnancy with resorption of one or more of the embryos may be even more common.
  • Congenital Malformations: anomalies are approximately twice as common in twin infants as in singleton infants and four times as common in triplets, it is reported the major malformations was 2.12% and minor malformations 4.13%. monozygotic twins have twice the incidence of fetal malformation compared with dizygotic twins. Conjoined twins are a rare complication of monozygotic twinning.
  • Hyperemesis Gravidarum: excess nausea and vomiting is common with multiple pregnancy, may be associated with the increased hormonal levels. The treatment is the same as for singleton pregnancy.
  • Anemia: maternal blood volume in twin pregnancy is approximately 500 ml greater than in singleton pregnancy and of course the fetal demands are proportionately greater. Thus, iron and folate deficiency anemias are more common. Iron supplementation of 60 to 80 mg and folic acid supplementation of 1 mg per day are recommended in multiple pregnancies, and along with high-protein diet.
  • Pregnancy Induced Hypertension: in multiple pregnancies it is more common and more severe. Most studies suggest it is three times more as compared to singleton.
  • Polyhydramnios: acute polyhydramnios does occur, particularly with monoamniotic twins. It has been reported as occurring in 5 to 8% of multiple pregnancies, and before 28 weeks the incidence has been reported as 1.7% of all twin pregnancies.
  • Preterm Delivery: the incidence of preterm delivery (before 37 weeks) in twin gestations approaches 50%. This incidence is 12 times higher than seen in singleton pregnancy and average length of gestation decreases inversely with the number fetuses present. Management of preterm labor is quite similar to that of singleton pregnancy.
  • Intrauterine Growth Restriction: two-thirds of twin infants show clinical and objective sign of intrauterine growth restriction. Approximately 70% of the infants born of multiple gestations are significantly growth restricted.
  • Twin-to-Twin Transfusion: approximately 15% of monochorionic twin pregnancies show clinical evidence twin transfusion syndrome. In these cases perinatal mortality is very high in both twins.
  • Hemorrhage: there is slight increase in antepartum hemorrhage and incidence of placenta previa; accidental hemorrhage and vasa previa is higher.
  • Neurological Damage: cerebral palsy, microcephaly, porencephaly, and multiple encephalomalacia occur more frequently in multiple pregnancies than singleton pregnancies and more frequently when delivery is preterm. Monochorionic infants have a significantly higher incidence of antenatal necrosis of the cerebral white matter than dichorionic infants or singleton.

Antepartum Management:

  1. Nutritional Considerations: it is recommended that maternal dietary intake in a multiple gestation be increased daily by approximately 300 kcal above that for a singleton pregnancy. Although optimal weight gain for women with multiple gestations has not been determined, it has been suggested that women with twins gain 35-45 pounds.
  2. Ultrasonography: it is useful in both prenatal diagnosis and antepartum surveillance. With its use, less than 10% of twin gestations are undiagnosed before labor and delivery. A detailed ultrasound evaluation of a multiple gestation should be performed during the second trimester. Determination of chorionicity is most accurate in the first trimester; as the pregnancy progresses, it becomes less accurate. Until the third trimester, twins follow the same growth curves that apply to singleton pregnancies. Evaluation of serial fetal growth should include estimated fetal weight of each fetus and appropriate and concordant interval growth.
  3. Routine Cervical Evaluation: either by clinical or ultrasonographic assessment has been investigated as an approach to predict preterm birth in the multiple gestations. Weekly digital cervical examination for clinical assessment of the cervix has not been associated with adverse maternal and fetal outcome. Vaginal Ultrasonography has been used to measure cervical length, width and funneling and to examine the relationship of these measurements to the risk of preterm birth.
  4. Bed Rest: the role and value of bed rest at home or in the hospital in the prevention of preterm delivery of a multiple gestation remains controversial. Antepartum hospitalization may be necessary in the multiple gestations for managing complications such as preterm labor, toxemia of pregnancy and abnormal fetal growth.
  5. Antepartum Surveillance: the routine use and benefit of antepartum fetal surveillance in the uncomplicated multifetal gestation has not been shown to be of benefit. When intrauterine growth restriction, abnormal fluid volumes, growth discordance, pregnancy-induced hypertension, fetal anomalies, monoamnionicity, or other pregnancy complications occur, fetal surveillance non-stress test and bio-physical profile is indicated. The use of home uterine activity monitor has been advocated by some to prevent or manage preterm labor in singleton or multiple gestations.
  6. Death of One Twin: once a single fetal demise is diagnosed, the gestational age of the pregnancy and the condition of the surviving fetus will dictate clinical management. When death of one fetus occurs prior to 34 weeks of gestation, fetal movement counting and increased antepartum fetal surveillance of the surviving twin should be undertaken. Fetal compromise or the presence of fetal lung maturation suggests a need for delivery. Maternal consumptive coagulopathy with hypofibrinogenemia occasionally may develop under these circumstances. Delivery may be appropriate if the death of one of twin occurs after 34 weeks of gestation.

Intrapartum Complication:

Malpresentation, cord prolapse, cord entanglement (particularly in monoamnionic twins), in coordinate uterine action, fetal distress, and surgical intervention are all more common during labor in multiple gestation than in singleton gestation. Locking of twins is also seen.


The ideal time of delivery for uncomplicated pregnancies is uncertain; however, if elective delivery is considered before 38 weeks of gestation, fetal lung maturity should be assessed. Ideally women with multiple pregnancies should undergo delivery by 40 weeks of gestation.

Labor and Delivery:

When a woman with known or suspected multiple gestations presents in labor, confirmation as soon as possible by ultrasound examination of fetal number and presentations is indicated. Both twins should be monitored continuously during labor. Ultrasonography should be available to determine the heart rate of the second twin as well as its orientation following delivery of the first twin. Appropriate experienced pediatric and anesthesia personnel should be notified and available at delivery. Capability for emergency cesarean delivery is necessary and blood should be available because the likelihood of operative intervention, as well as postpartum hemorrhage, is increased.

Route of Delivery:

Delivery should be based on individual needs and may depend on the clinician's practice and experience. The various twin presentations should be taken into account.
Twin A-Vertex with Twin B-Vertex: vaginal delivery is anticipated most of the time. Cesarean delivery should only be performed for the same indications applied to singleton gestations.
Twin A-Vertex with Twin B-Nonvertex: cesarean delivery, however, is not always necessary. Vaginal delivery of twin B in the nonvertex presentation is a reasonable option for a neonate with an estimated weight greater than 1,500 g. There are insufficient data to advocate a specific route of delivery (vaginal or abdominal) of twin B whose birth weight is less than 1,500 g, although cesarean delivery is performed frequently.
Twin A-Nonvertex: in general, cesarean delivery is the method of choice when the first twin is nonvertex, such as a breech or transverse presentation. The safety of vaginal delivery for this group has not been documented. When the first twin is breech and the second is in a vertex presentation, the possibility of locked twins exists.

Interval Between Deliveries:

The interval between delivery of each twin is not critical in determining the outcome of twin B. Surveillance of twin B with real time Ultrasonography and continuous monitoring of the fetal heart rate, however, are advised after the delivery of twin A. Rapid delivery may be required because of complications, such as abruption placentae, cord prolapse, or a decrease in the fetal heart rate. If labor had not resumed in reasonable time after the delivery of twin A, oxytocin augmentation with careful fetal heart rate surveillance can be initiated. Once the vertex is in the pelvic inlet, amniotomy can be performed. When fetal condition dictates the need to expedite delivery, internal podalic version and breech extraction may be an acceptable alternative. Obstetricians should select the delivery technique with which they are most comfortable.


In conclusion, multiple pregnancies require early detection and identification of number of pregnancies, early detection of the complications and their proper management. Premature babies need prolonged and expensive care. Patient education and availability of trained healthcare providers in the area can reduce the mortality and morbidity. There are support groups for the parents of multiple births available at almost all the area hospitals, which deal with high-risk deliveries.

Suggested Readings:

  1. Malone FD, D'Alton ME. Multiple gestation, In: Creasy, RK Resnik R. editors. Maternal-fetal medicine: principles and practice, 5th ed. Philadelphia (PA): Saunders; 2004: 513-536
  2. ACOG Practice Bulletin. Multiple gestation: complicated twin, triplet, and high-order multifetal pregnancy. Number 56. October 2004
  3. Simoes T, Amaral N, Lerman R et al. Prospective risk of intrauterine death of monochorionic-diamniotic twins. Am J Obstet Gynecol 2006;195:134-139
  4. Lee YM, Cleary-Goldman J, Thaker HM et al. Antenatal sonographic prediction of twin chorionicity. Am J Obstet Gynecol 2006;195:863-867
  5. Miura K, Nikawa N. Do monchorionic dizygotic twins increase after pregnancy by assisted reproductive technology? J Hum Genet 2005;50:1-6
  6. Kahn B, Lumey LH, Zybert PA et al. Prospective risk of fetal death in singleton, twin, and triplet gestations: implications for practice. Obstet Gynecol 2003;102:685-692
  7. Silver RM, Varner MW, Reddy U et al. Work-up of stillbirth: a review of the evidence. Am J Obstet Gynecol 2007;196:433-444
  8. Lee YM, Wylie BJ, Simpson LL et al. Twin chorionicity and the risk of stillbirth. Obstet Gynecol 2008;111:301-308

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