Evolution & the Cesarean Section Rate

Abstract

Nothing in biology makes sense except in the light of evolution. This was the title of an essay by geneticist Theodosius Dobzhansky writing in 1973 (Dobzhansky, 1973). Many causes have been given for the increased Cesarean section rate in developed countries, but biologic evolution has not been one of them. The C-section rate will continue to rise, because the ability to perform a safe C-section has liberated human childbirth from selection directed against too small a maternal pelvis and too large a fetal head. Babies will get bigger and pelves will get smaller because there is nothing to prevent it. The Present The World Health Organization (WHO) estimates that there are 529,000 maternal deaths per year worldwide (WHO, 2005). Only 1% of these occur in the developed world. Eighty percent of these are maternal deaths due directly to obstetrical factors, and the leading causes in descending order are hemorrhage, infection, eclampsia, and obstructed labor. Obstructed labor accounts for 8% of deaths, or 42,000 women, worldwide. This relative percentage is higher in certain regions of the world, for example up to 35% in Nigeria (AbouZahr, 1998). It has been estimated from historical data, and from experience with certain religious groups that decline modern medical interventions, that the natural maternal mortality rate is 1,000-1,500 per 100,000 births (Van Lerberghe & De Brouwere, 2001) but even this underestimates maternal mortality in certain regions such as Badakshan, Afghanistan (6,500 per 100,000 births with the leading cause of death being obstructed labor) (Bartlett et al., 2002). That this mortality can be diminished by interventions has been demonstrated in Zaire where nurses were taught to perform emergency C-sections and symphysiotomies (White el al., 1987) and in Tanzania where hospital-based intervention programs reduced maternal mortality due to uterine rupture by 70% (Mbaruku & Bergstrom, 1995). Evolution by selection requires phenotypic variation in a population, genetic heritability of this variation, and selection forces. Clearly selection continues to act in some parts of the world to limit extreme variation in maternal pelvic dimensions and fetal head size. There is limited population data on maternal pelvic dimensions. Since newborn head volume in relation to newborn birth weight is constant among the great apes and man (Wilcox, 1983), newborn birth weight can be used instead of head size to trace the evolution of head-pelvis compatibilities. Birth weights have been shown to be statistically normally distributed (Wilcox, 1983). One prediction of the hypothesis that evolution accounts for some component of the rise in the C-section rate would be an increase in birth weight over time. There is evidence that birth weights are increasing over time in the United States, although different populations by geographic location and race have been examined (Williams, 1975; Brenner et al., 1976; Lubchenco et al., 1963; Ott, 1993; Alexander et al., 1996). Data from Brenner and Amini compare a similar study population in the same geographic area over 18 years, and there is a shift to the right by 40 grams of the curve for the birth weight at 40 weeks (Brenner et al., 1976; Amini et al., 1994). This is only a 1.2% increase, but is a quite significant jump in terms of one measure of evolutionary change, that of the darwin, which is a 1% quantitative change in a phenotypic measure over one million years. Birth weight data gathered by the United States Bureau of Vital Statistics is not correlated to gestational age, but comparisons between 1960 and 1997 show that there is a 2% increase in the percentage of babies born weighing 3,500-3,999 grams and a 1% increase in babies weighing 4,000-4,499 grams with stable preterm birth rates (U.S. Bureau of Vital Statistics, 1960, 1997). These weight intervals would most likely be term infants. Increase in birth weight does not prove that evolution is occurring, since only the potential phenotype is determined by the genotype, such that environmental factors can affect final phenotypic appearance. …