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Master of Science (M.S.)
Year Degree Awarded
Month Degree Awarded
Maternal Age at Delivery, Birth Order, Secondary Sex Ratio
The observation that more boys were born than girls was noted in the late 1660’s. Recent studies suggest the secondary sex ratio (SSR) is declining in industrialized countries. SSR is proposed as a sentinel for reproductive health. Declining SSR may reflect environmental factors or other influences of reproductive outcomes. We evaluated maternal age, birth order and SSR in the Old Order Amish (OOA), a homogenous sub-group with large family sizes. We used data from the Anabaptist Genealogy Database consisting of records for live births from 1696-2003. We used t-tests to compare mean maternal age and birth order by offspring sex, ANOVA to evaluate whether SSR has changed over time, and logistic regression for multivariable models. We evaluated clustering of SSR within families using random effects models and likelihood ratio tests of random effects. Maternal age was not associated with SSR (OR=1.003 [95% CI, 0.995-1.010), even after adjusting for birth order (AOR=1.000 [95% CI, 0.989-1.012). Similarly, we did not find an association between birth order and SSR in both unadjusted models (OR=1.007 [95% CI, 0.991-1.022), and those adjusted for maternal age (AOR= 1.006 [95% CI, 0.982-1.032]). The proportion of male births varied, however, there was no significant trend overtime. Lastly, we found a significant random effect (P<0.05), which may provide indication that having male births is heritable in families. Conclusions: Neither maternal age nor birth order is associated with the sex of an offspring. These findings suggest that decreases in SSR are unrelated to demographic factors, and rather may be related to other factors such as environmental exposures or other xenobiotic chemicals. These results may be relevant in providing information to the leading indicators to the decline in SSR.
Brian W. Whitcomb