Mother has the final say.

Grant et al., in an article on p. 812 of the May issue of Biology of Reproduction, find that the sex of bovine embryos positively correlates with pre-ovulatory follicular testosterone. How exposure of oocytes to follicular testosterone may influence the ability of a metaphase II egg to be preferentially inseminated by a Y-bearing sperm remains a mystery. What is known is that chronic stress elevates testosterone in females (but not males). So if you would like your next sibling to be a sister, don’t aggravate your mother.-Society for the Study of Reproduction