CHAPTER 2

LECTURE EXTENSION

Recent Trends in Multiple Births in the United States

The text provides a brief overview of multiple births and the factors associated with twins, triplets, and other higher order multiple deliveries. A recent report from the National Center for Health Statistics (Ventura, Martin, Curtin, & Mathews, 1998) uses data from the births certificates of the 3.9 million births that occurred in 1997 to summarize these trends in greater detail. The number of multiple deliveries climbed to 110,874 for 1997 and included 104,137 twins, 6,148 triplets, 510 quadruplets, and 79 quintuplets and other higher order multiple births, representing a 3-percent increase in the number of twins, and a 16-percent jump in the number of triplets births over 1996. Since 1980, the number of twins has risen 52 percent (from 68,339) and the number of triplets and higher order multiple births by 404 percent (from 1,337).

Put another way, one in every 576 births was a triplet or other higher order multiple in 1997, compared with about one of every 2,700 births in 1980. Older maternal age (women in their thirties are more likely to have a multiple birth than those in their twenties, even without the use of fertility therapy), and the more widespread use of fertility -enhancing therapies (fertility drugs and techniques such as in vitro fertilization) have been associated with the unprecedented rise in multiple births.

Twinning rates rose among non-Hispanic white, non-Hispanic black, and Hispanic women for 1996 to 1997, but continue to be highest for non-Hispanic black women. In contrast, and despite substantial increases in higher order multiple births among non-Hispanic black women (64 percent) and Hispanic women (93 percent) since 1991, non-Hispanic white women were more than twice as likely as Hispanic women to have a triplet, quadruplet, or quintuplet birth (230.8 compared with 90.0 and 72.7 per 100,000, respectively) in 1997. Non-Hispanic white mothers tend to be older, and are more likely than their black and Hispanic counterparts, to seek infertility services.

Multiple births rates generally rise with increasing maternal age (there is a slight dip at age 40 to 44 years), with rates peaking for older mothers 45 to 54 years of age. For 1997, 16.6 percent of births to mothers aged 45 years and over was a multiple, a level 10 times higher than that for mothers under 20 years of age (1.5 percent) and more than 3 times higher for women in their thirties (3.9 percent).

There are risks associated with multiple births. Multiple births are more likely than singletons to be born preterm (57 versus 10 percent) or low birth weight (56 versus 6 percent). Multiple comprised only 3 percent of all births in 1997, but 21 percent of all low birth weight infants and 14 percent of preterm infants.

Ventura, S. J., Martin, J. A., Curtin, S. C., & Mathews, T. J. (1998). Births: Final data for 1997. National Vital Statistics Reports (Vol. 47, No. 18). Hyattsville, MD: National Center for Health Statistics.

LEARNING ACTIVITY

Analyzing a Pedigree

Genetic counselors often construct a pedigree of a family to estimate the likelihood that children will display a trait or be carriers of it. In a pedigree, males are indicated by squares and females are represented by circles. Individuals affected by a certain trait are colored in. A horizontal line is used between symbols to represent mating. The symbols connected to the mating line by vertical and horizontal lines represent the children resulting from the mating in terms of birth order (from left to right).

The diagram below represents a pedigree of a trait. Determine whether the gene in question is behaving as a dominant or recessive gene. Identify the genotypes of as many individuals in the pedigree as possible. Also, estimate the likelihood, in terms of probabilities, of the trail appearing in other children in the family.