Maternal age is another important risk factor for multifetal pregnancies (Fig. 45-3). Dizygotic twinning frequency rises almost fourfold between the ages of 15 and 37 years (Painter, 2010). As such, there is a paradox of declining fertility but increasing twinning rates with advancing maternal age (Beemsterboer, 2006). Another explanation for the dramatic rise in twinning with advancing maternal age may be a higher use of ART in older women (Ananth, 2012). Paternal age has also been linked to twinning frequency, but its efect is felt to be small (Abel, 2012). Although twin pregnancy is associated with greater risks for most adverse perinatal outcomes, McLennan and associates (2017) did not find advanced maternal age to be an additional risk factor for fetal and infant death. From this population-based study of the United States, they concluded that women in their 30s may be counseled that their age is not a major additional risk factor for adverse obstetric outcomes in the setting oftwin pregnancy.
Increasing pariy independently raises the incidence of twinning in all populations studied. During a 30-year period, Antsaklis and coworkers (2013) noted a progressively increasing positive correlation between multiparity and twinning. However, they
FIGURE 45-3 Multifetal birth rates in the United States according to maternal age and race, 2015. (Data from Martin, 201o7.) cautioned that greater use ofRT may be partially contributory. In a two-year study from Nigeria, where such technology is not commonly available, Olusanya (2012) calculated the efects of multiparity compared with primiparity. They found an eightfold rise in multifetal gestation rates when parity was ;4, and a 20-fold rise when parity was :5.
As a determinant of twinning, the family history ofthe mother supersedes that of the father. One study of 4000 genealogical records showed that women who themselves were a dizygotic twin gave birth to twins at a rate of 1 set per 58 births (White, 1964). Women who were not a twin, but whose husbands were a dizygotic twin, gave birth to twins at a rate of 1 set per 116 pregnancies. Painter and associates (2010) performed genomewide linkage analyses on more than 500 families of mothers of dizygotic twins and identified four potential linkage peaks. he highest peak was on the long arm ofchromosome 6, and other suggestive peaks were on chromosomes 7, 9, and 16. That said, the contribution of these variants to the overall incidence of twinning is likely small (Hoekstra, 2008).
In animals, the litter size number grows in proportion to nutritional suiciency. Evidence from various sources indicates that this occurs in humans as well. Nylander (1971) showed an increasing gradient in the twinning rate related to greater nutritional status as relected by maternal size. Taller, heavier women had a twinning rate 25 to 30 percent greater than short, nutritionally deprived women. Likewise, Reddy and associates (2005) found an association of maternal weight and dizygotic twinning in the
United States, in the absence of fertility drugs. Indeed, the influ ence of maternal weight s a factor for twinning will continue to rise in importance s the percentage of obese women in the United
States continues to grow.