monozygotic (MZ) if one egg was. Monozygotic twins result when the fertilized and rapidly dividing egg splits in two within days after conception. The term identical is often used to describe MZ twins, and fraternal denotes DZ twins. These terms are misleading, so the preferred terms are monozygotic and dizygotic.86 About 70% of twins are DZ, and 30% are MZ.
Monozygotic twins are always the same sex, whereas DZ twins are the same sex half the time. Monozygotic twins are genetically identical in almost all ways, but they are seldom absolutely identical. Genetic differences in pairs of MZ twins can result from chromosome abnor-malities in one twin, unequal genetic expression of ma-ternally and pama-ternally derived genes, and environmental effects on gene expression. Rates of MZ twins are remark-ably stable across population groups and do not appear to be influenced by heredity.87
Dizygotic twins represent individuals with differ-ing genetic “fdiffer-ingerprints.” The incidence of DZ twin pregnancies is influenced both by inherited and envi-ronmental factors. Rates of DZ twins vary among ra-cial groups and by country. Rates tend to decrease in populations during famine and to increase when food availability and nutritional status improve.88 Pericon-ceptional vitamin and mineral supplement use has also been related to an increased incidence of DZ twin pregnancy.89
Twins also vary in the number of placentas; some are born having used the same placenta, but more com-monly each fetus has its own. Twins may share a common amniotic sac and one of the membranes around the sac (the chorion), or have separate amniotic sacs and mem-branes (Illustration 5.4). Twins at highest risk of death, malformations, growth retardation, short gestation, and other serious problems are those that share the same (referred to as triplet 1 births)
increased from 1 in 2941 to 1 in 653 births.83 The leading reason for the increased prev-alence of multifetal pregnan-cies in the United States and other developed countries is the use of assisted reproduc-tive technology. Rates of twin and higher-order births are highest by far in women 45–54 years old (1 in 5 births), the age group most likely to receive assisted reproductive tech-nological interventions to achieve pregnancy.83
The progressively older ages at which U.S. women are bearing children also contribute to rising rates of multifetal pregnancies. The chances of a spontaneous multifetal preg-nancy increase with age after about 35 years. Rates of spon-taneous multifetal pregnancy also increase with increasing weight status. For example, the rate of twin pregnancy is about two times higher in obese than in underweight wom-en.84 Rates of triplet 1 pregnancies are headed downward due to improved assisted reproductive technologies that re-duce higher-order multifetal pregnancies.82
Upward trends in low birth weight and preterm de-livery in the United States over recent years have been strongly influenced by the upsurge in multiple births. Only 3% of newborns are from multifetal pregnancies, yet they account for 21% of all low-birth-weight newborns, 14%
of preterm births, and 13% of infant deaths.85
Background Information about Multiple Fetuses
The most common type of multifetal pregnancies, those with twin fetuses, come in several types and levels of risk.
Twins are dizygotic (DZ) if two eggs were fertilized, and Assisted Reproductive Technology
(ART) An umbrella term for fertility treatments such as in vitro fertilization (IVF, a technique in which egg cells are fertilized by sperm outside the woman’s body), artificial insemination, and hormone treatments.
Placenta
Amniotic sac Chorion
Uterus
Illustration 5.4 Variations in amniotic sacs, chorions, and placentas in twin pregnancy. Drawing (a) shows twins with two amniotic sacs, two chorions, and two placentas. Drawing (b) represents twins sharing one amniotic sac, chorion, and placenta, and drawing (c) shows twins with two amniotic sacs, one chorion, and two placentas that have grown into one.
source: Schematic representations drawn by the author with the help of Scott Strachan, 2009.
(a) (b) (c)
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Illustration 5.5 Rates of fetal weight gain in single-ton, twin, and triplet fetuses.
source: From “Multifetal pregnancies: Epidemiology, Clinical Characteristics, and Management,” by M. Smith-Levitin et al., in Maternal-Fetal Medicine: Principles and Practice, 3rd Ed., R.K. Creasy and R. Resnick (Eds.), p. 589–601. Copyright © 1994. Reprinted by permission of W. B. Saunders Co.
Gestation (Weeks)
Mean Weight (kg)
4
3
2
1
20 24 28 32 36 40
Singletons
Twins Triplets
Table 5.12 Risks to mother and fetuses associated with multifetal pregnancy92
Pregnant Women Preeclampsia
•
Iron-deficiency anemia
•
Gestational diabetes
•
Hyperemesis gravidarum
•
Placenta previa
•
Kidney disease
•
Fetal loss
•
Preterm delivery
•
Cesarean delivery
• Newborns
Neonatal death
•
Congenital abnormalities
•
Respiratory distress syndrome
•
Intraventricular hemorrhage
•
Cerebral palsy
•
Table 5.13 Average birth weight and gestational age at delivery, and low-birth-weight rates, of singleton, twin, and triplet newborns92,93
Mean Birth Weight
Mean Gestational
Age
Low-Birth-Weight Rate Singletons 3440 g
(7.7 lb)
39–40 weeks 6%
Twins 2400 g
(5.4 lb)
37 weeks 54%
Triplets 1800 g (4.0 lb)
33–34 weeks 90%
amniotic sac and chorion, and to a lesser degree, MZ twins in general.87
Determining twin type is not always an easy task dur-ing or after pregnancy. Definitive diagnoses of tough cases can be made through DNA fingerprinting.90
In Utero Growth of Twins and Triplets Fetal growth patterns of twins and triplets compared to sin-gleton fetuses are shown in Illustration 5.5. Rates of weight gain for each group of fetuses are the same until about 28 weeks of gestation. Rates of weight gain begin to decline in twin and triplet fetuses after that point, however, and remain lower until delivery. Variations in birth weight of twin and triplet newborns appear to be related to factors that affect fetal growth after 28 weeks of pregnancy.90
The Vanishing Twin Phenomenon The disappearance of embryos within 13 weeks of conception is not unusual.
It has been estimated that 6 to 12% of pregnancies begin as twins, but that only about 3% result in the birth of twins. Most fetal losses silently occur by absorption into the uterus within the first 8 weeks after conception. The prognosis for continued viability of a pregnancy associ-ated with a vanishing twin tends to be good.91
Risks Associated with Multifetal Pregnancy
Singleton pregnancy is the biological norm for humans, so it may be expected that multifetal pregnancy would be accompanied by increasing health risks (Table 5.12).82 Multifetal pregnancies present substantial risks to both mother and fetuses, and the risks increase as the number of fetuses increases (Table 5.13). Newborns from twin pregnancies at lowest risk of death in the perinatal period weigh between 3000 and 3500 grams (6.7 to 7.8 lb) at
birth and are born at 37–39 weeks gestation. Triplets tend to do best when they weigh over 2000 grams (4.5 lb) and are born at 34–35 weeks gestation.94
Unfortunately, these outcomes do not represent the usual. Data presented in Table 5.14 on the next page show that median weights of twins born at 37, 38, and 39 weeks gestation fall below the 3000- to 3500-gram range.
However, the 3000- to 3500-gram birth weight range for twins, and the .2000-gram mark for triplet newborns, can serve as goals for the provision of nutrition services.
Interventions and Services for Risk Reduction
Special multidisciplinary programs that offer women with multifetal pregnancy a consistent, main provider of care; preterm prevention education; increased attention to nutritional needs; and intensive follow-up achieve better pregnancy outcomes than does routine prenatal care.93 Rates of very low birth weight (≤1500 g or ≤3.3 lb) have
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Rate of Weight Gain in Twin Pregnancy A positive rate of weight gain in the first half of twin pregnancy is strongly associated with increased birth weight.92 On the other hand, weight loss after 28 weeks of pregnancy in-creases the risk of preterm delivery by threefold.100
Reasonable rates of weight gain for women with twin pregnancy are:
0.5 pounds (0.2 kg) per week in the first trimester
●
1.5 pounds (0.7 kg) per week in the second and
●
third trimesters92
Weight Gain in Triplet Pregnancy Several studies have examined the relationship between weight gain and birth weight in women with triplets. The general result is that weight gains of about 50 pounds (22.7 kg) corre-spond to healthy-sized triplets. Rates of gain related to a total weight gain of 50 pounds in women who will average 33 to 34 weeks of gestation are 1.5 pounds (0.7 kg) per week or more, starting as early in pregnancy as possible.92
Dietary Intake in Twin Pregnancy
Ensuring “adequate nutrition” is widely acknowledged to be a key component of prenatal care for women with mul-tifetal pregnancy. However, it is not clear what constitutes adequate nutrition. Energy and nutrient needs clearly in-crease during multifetal pregnancy due to inin-creased levels of maternal blood volume, extracellular fluid, and uterine, placental, and fetal growth. The normally high expansion in extracellular volume and its side effect of leg and an-kle edema can be seen in the healthy woman with a twin pregnancy shown in Illustration 5.6. Increases in energy and nutrient needs place demands on the mother in terms of the nutritional costs of building and maintaining these tissues. Although their newborns are smaller, women with twins still produce around 5000 g (11.2 lb) of fetal weight, and women with triplets 5400 g (13.4 lb) or more.
Evidence of higher caloric need for tissue mainte-nance and growth in multifetal than singleton pregnancy comes from studies that show increased weight gain and a quicker onset of starvation metabolism in women expecting more than one newborn. Reduced rates of twin been reported to be substantially lower (6% versus 26%),
neonatal intensive care admissions three times lower (13% versus 38%), and perinatal mortality strikingly lower (1% versus 8%) among women who receive such services.95 Interventions offered by the Montreal Diet Dis-pensary, which focuses on improving the nutritional status and well-being of the pregnant women served, have been shown to substantially reduce poor outcomes compared to those for similar women not receiving the services. Im-provements include a 27% reduction in the rate of low birth weight, 47% decline in very low birth weight, 32%
lower rate of preterm delivery, and a 79% drop in mortal-ity during the first 7 days after birth.97
Nutrition and the Outcome of Multifetal Pregnancy
Nutritional factors are suspected of playing a major role in the course and outcome of multifetal pregnancy, but much remains to be learned. Of the nutritional factors that influence multifetal pregnancy, weight gain during twin pregnancy has been studied most.
Weight Gain in Multifetal Pregnancy As with single-ton pregnancy, weight gain in multifetal pregnancy is linearly related to birth weight, and weight gains associated with newborn weight vary based on prepregnancy weight status (Table 5.15).98 The Institute of Medicine recommends that women with twins gain 25 to 54 pounds (11.4 to 24.5 kg).
It is provisionally advised that normal weight women gain toward the upper end of this range, and overweight and obese women closer to the lower end.99
Table 5.14 Median birth weight for gestational age at delivery of twins
Gestational
Age, Weeks Birth Weight
28 995 g (2.2 lb)
29 1145 g (2.6 lb)
30 1300 g (2.9 lb)
31 1445 g (3.2 lb)
32 1580 g (3.5 lb)
33 1750 g (3.9 lb)
34 1905 g (4.3 lb)
35 2165 g (4.8 lb)
36 2275 g (5.1 lb)
37 2430 g (5.4 lb)
38 2565 g (5.7 lb)
39 2680 g (6.0 lb)
40 2810 g (6.3 lb)
41 2685 g (6.0 lb)
source: Data from Cohen SB, 1997.96
Table 5.15 Prepregnancy weight status and weight-gain relationships in twin pregnancy84
Prepregnancy Weight Status
Weight Gain Related to Birth Weights of
.2500 g (5.5 lb)
Underweight 44.2 lb (20.1 kg)
Normal weight 40.9 lb (18.6 kg)
Overweight 37.8 lb (17.2 kg)
Obese 37.2 lb (16.9 kg)
Very obese 29.2 lb (13.3 kg)
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Vitamin and Mineral Supplements and Multifetal Pregnancy Benefits and hazards of multivitamin and mineral supplement use in multifetal pregnancy have not been reported. Consequently, the extent to which they may be required is unknown. Levels of nutrient intake exceeding the DRI Tolerable Upper Intake Levels should be avoided.
Nutritional Recommendations for Women with Multifetal Pregnancy
Due to the lack of study results, nutritional recommen-dations for women with multifetal pregnancy are largely based on logical assumptions and theories (Table 5.16). It is reasoned, for example, that caloric needs for twin preg-nancy can be extrapolated from weight gain. Theoretically, to achieve a 40-pound (18.2 kg) weight gain, or 10 pounds (4.5 kg) more than in singleton pregnancy, women with twins would need to consume approximately 35,000 cal more during pregnancy than do women with singleton pregnancies. This increase would amount to about 150 cal per day above the level for singleton pregnancy, or an aver-age of 450 cal more per day than prepregnancy. To achieve higher rates of gain, underweight women may need a higher level of intake, and overweight and obese women lower levels. Energy needs will also vary by energy expend-iture levels. As for singleton pregnancy, adequacy of caloric intake can be estimated by weight-gain progress.92
Food-intake recommendations for women with multifetal pregnancy are primarily estimated based on assumptions related to caloric and nutrient needs. Women with multifetal pregnancy likely benefit from diets selected from the MyPyramid groups and nutrient intakes that somewhat exceed the RDAs/AIs.
Although twin pregnancies are higher-risk than single-ton pregnancies, outcomes of twin pregnancy can be excel-lent. Illustration 5.7 shows healthy, term newborn twins.
Their mother remained in good health during pregnancy while consuming the type of diet and supplements and gain-ing weight as recommended by her health care providers.
Recommendations from the Popular Press Web-sites, books, and pamphlets are available that provide ample amounts of scientifically unsupported “guesses”
about food and nutrient requirements of women with multifetal pregnancy. Even if presented with steely reso-lution, any advice that strays from current scientifically based wisdom about nutritional needs of women during pregnancy should be sidestepped.