When two fetuses are carried simultaneously and delivered by a woman, it is termed a twin birth. Most multiple births are twins. In 1997, 94% of multiple births reported were twins. The numbers of twin, triplet, and higher-order multiple births have climbed at an unprecedented rate during the past 20 years. Between 1980 and 1997, the number of live births in twin deliveries rose 52%. By contrast, singleton births, or one baby delivered alone, rose only 6% during this time.
Twins referred to as fraternal or nonidentical (also known as dizygotic or binovular) are much more common than paternal or identical twins. They are derived from the development of two eggs that are released from separate follicles at about the same time. They are fertilized by different sperm and become implanted in different places in the uterus. Only rarely do they share the same placenta, and this occurrence is thought to be possibly caused by the use of in vitro fertilization. The embryos have different chromosomal makeups and may or may not be the same gender. They may be as similar or different in characteristics as siblings born at separate times.
The incidence of multiple births in white women has increased more rapidly than among African American women. Because of this, the historical differences in the rates of multiple births between them have been largely erased. For 1997, the twin birth rate for Hispanic women (19.5 per 1,000 births) was substantially less than for non-Hispanic white or African American women (28.8 and 30.0, respectively). The Asian countries of Japan and China have the lowest twinning rates, estimated at 1 in 150 and 1 in 300, respectively.
There are numerous explanations for the increase in twin births. There has been a growing trend for women to begin their childbearing at a later age and to continue to reproduce into later years. The likelihood of multiple births increases steadily with advancing maternal age. A woman in her 30s has twice the chance of giving birth to fraternal twins as her counterpart 10 years younger. Seventeen percent of mothers older than 45 give birth to twins. Of women older than 50, nearly one in nine gives birth to twins. For mothers who have already had one set of fraternal twins, the chances of conceiving another set are four times greater than the average woman, or about 1 in 12. Improvements in the early detection of twin gestations through the use of ultrasound technique, improved prenatal care, and medical advances in the care of neonates have increased the survival rate of twins. Advances in infertility treatments have also contributed significantly to the increase.
All multiple pregnancies are automatically considered high risk. Twins are eight times more likely than singletons to be born at a weight of less than 1,500 grams, or very low birthweight (VLBW). More than one half of all twins are born with low birth weight (LBW; less than 2,500 grams). This compares with only 6% of singletons born with LBW. Additional risks for these infants include prematurity; underdeveloped lungs, which can lead to respiratory distress syndrome (RDS); cerebral palsy; hearing or vision problems; and developmental delays or learning disabilities. Risks for the mother include preeclampsia (a rapid rise in blood pressure, protein in urine, and fluid retention), anemia (low red blood count), preterm bleeding, preterm labor, gestational diabetes, blood clots, and miscarriage. There are thought to be many more twin conceptions than births because often one or both fetuses are lost during the first trimester owing to miscarriage or vanishing twin syndrome.
As would be expected, families of twins face increases in financial burdens, marital stress, child care issues, and a multitude of other adjustments. With the ever-growing incidence of twins, more and more families are affected.
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- Keith, G., Oleszczuk, J. J., & Keith, D. M. (2000). Multiple gestation: Reflections on epidemiology, causes and consequences. International Journal of Fertility, 45(3), 206–214.
- Keith, L. , Papiernik, E., Keith, D. M., & Luke, B. (Eds.). (1995). Multiple pregnancy—Epidemiology, gestation, and perinatal outcome (pp. 163–190). New York, London: Hecht.
- Martin, J. , & Park, M. M. (1999). Trends in twin and triplet births: 1980–97. National Vital Statistic Reports, 47(24). National Organization of Mothers of Twins Clubs, http://www.nomotc.org
- Paternity Angel, http://www.paternityangel.com
- Twin Stuff, http://www.twinstufcom
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