NCBI Bookshelf. Sex differences of importance to health and human disease occur throughout the life span, although the specific expression of these differences varies at different stages of life. Some differences originate in events occurring in the intrauterine environment, where developmental processes differentially organize tissues for later activation in the male or female. In the prenatal period, sex determination and differentiation occur in a series of sequential processes governed by genetic and environmental factors.
Fetal Development: Baby's Reproductive System and Sex Organs
The development of the reproductive systems begins soon after fertilization of the egg, with primordial gonads beginning to develop approximately one month after conception. Reproductive development continues in utero, but there is little change in the reproductive system between infancy and puberty. To become a male, an individual must be exposed to the cascade of factors initiated by a single gene on the male Y chromosome. Because females do not have a Y chromosome, they do not have the SRY gene. Without a functional SRY gene, an individual will be female. In both male and female embryos, the same group of cells has the potential to develop into either the male or female gonads; this tissue is considered bipotential. The SRY gene actively recruits other genes that begin to develop the testes, and suppresses genes that are important in female development.
The start of pregnancy is actually the first day of your last menstrual period. This is called the 'menstrual age' and is about two weeks ahead of when conception actually occurs. Each month a group of eggs called oocytes is recruited from the ovary for ovulation release of the egg. The eggs develop in small fluid-filled cysts called follicles.
The first third of your pregnancy is called the first trimester. It is made up of weeks 1 through 12 or 13 of pregnancy. Here is a summary of how your baby develops during the first trimester.