What are the odds of giving birth to a boy?
In most industrialized countries today, there are about 105 boys born for every 100 girls born, but this ratio isn't constant. Early in the twentieth century, there was an increase in the percentage of boys born. Male embryos and fetuses are more susceptible to loss in the womb, and so this increase was related to improved prenatal and obstetrical care during the early part of the twentieth century.
Male embryos are more vulnerable than female embryos. The reasons are largely unknown, but the fact that male embryos carry an X and a Y chromosome as opposed to two X's like female embryos may play a significant role in this. Because they carry two different chromosomes, genetic problems on one can't be compensated for by the other.
Yet despite this obstacle, boys were on the rise in the twentieth century. Since the 50's and 60's, the overall percentage of boys being born in the United States, Canada, and several European countries has been declining.
One theory is that the decline is in part a result of environmental contaminants. To what extent, it's difficult to tell just yet. Scientists do know, however, that drug use, occupational exposures, and environmental accidents that involve high exposures to toxic chemicals can dramatically decrease the percentage of boys born in a set population. Again, this might have something to do with males having two different chromosomes.