| Chapter 2: | Background on Population Sex Ratio |
|
|
This is a limited free preview of this book. Please buy full access.
Figure 2.1. Sex ratio by gestational age.
He noted in 1662, using parish records of births and deaths, that there were more male infants born annually in London than female infants, and he also documented that more male infants died during the 1st year of life than female infants.
It has been thought that timing of insemination affects the primary sex ratio (sex ratio of conceptions) and thus subsequently the sex ratio of births. Fertilization early in the cycle is theorized to yield a greater proportion of male fetuses, and has been proposed as an explanation for increased sex ratio at birth during and after the World Wars. Contrary to this theory, in a multicenter, prospective study of pregnancies among women using natural family planning, Gray et al. (1998), analyzed data from diaries of coitus and menses and found that the sex ratio among 947 singleton births did not vary consistently or significantly with the estimated timing of insemination relative to the day of ovulation, or with the estimated length of the follicular phase.
A lower sex ratio at birth was found among women with a vegetarian diet in a relatively small study of several thousand nonvegetarian and several hundred vegetarian women (Hudson & Buckley, 2000).