Sex Ratio

Abstract Sex of the baby is routinely recorded, and the ratio of boys to girls at birth is simple to calculate. A ratio of 51 boys to 49 girls has remained constant over time and across a variety of conditions. Environmental toxicants, war, and many other factors have been claimed to affect the sex ratio, but the only ones that can be reliably replicated are those with inconsequentially small effects. Sex selection is a priority for some couples, although there is no simple means of doing so. Best-selling books provide instructions on how couples might increase their chances of having a baby of one sex or the other. These methods are successful half the time. Sex selection is possible using more intensive methods, for example, by sorting X and Y sperm or by genetic identification of the desired sex among in-vitro conceptuses. Selective abortion of fetuses according to their sex on ultrasound exam is known to occur, even though it is illegal in all settings.