A baby’s genotypic gender is determined at conception. Genotypic gender in humans is XX (female) or XY (male), though some variations exist based on chromosomal anomalies (i.e. sex polysomies, such as XXY and XYY). The actual sex of the baby (girl or boy) is determined starting in the second month of embryonic development, usually based on the genotypic gender.
Genotype at conception
The process of conception in humans based on sexual reproduction is the fusion of one egg donated by the mother and one sperm donated by the father. In each of these sex cells, each parent donates half of the chromosomes the child will have: 22 autosomal chromosomes and a sex chromosome, for a total of 23 chromosome pairs in the embryo. The sex chromosome from the mother is always X because the female genotype is XX. The sex chromosome from the father may be X or Y, with a 50:50 chance under normal circumstances. Thus, the genotypic gender of the embryo and subsequent baby is determined at conception depending on which sperm fuses with the egg.
Phenotypic gender determination
The phenotypic gender is referred to as the baby’s sex. The physical manifestation of the genotype is the result of two biological processes: sex determination and sex differentiation. Sex is determined by a hierarchical genetic pathway based on whether the Y chromosome is present in the embryo. The presence of the Y chromosome drives testicular development around the tenth week of pregnancy. The Y chromosome has a dominant influence on gender phenotype, so that the presence of multiple X chromosomes does not prevent male phenotype development if a Y is present.
Multiple genes from both sex chromosomes, particularly the SRY region of the Y chromosome, contribute to the development of sex characteristics in the embryo. Hormones known as androgens and their receptors play a role in the development of these characteristics. The sex of the baby – whether it’s a boy or a girl – cannot be seen until the second trimester of pregnancy though the programming is present from conception.
Predicting and choosing gender
Some sites claim that there are gender prediction kits that can predict the baby’s gender prior to conception. This is not possible. Which sex chromosome a father donates is completely random unless undergoing in vitro fertilization or intrauterine insemination that used sperm sorting to influence the ratio of X and Y sperm. Even then, the chances of conceiving a baby with a particular genotypic gender is 76 to 91 percent. Other methods of influencing the gender of the baby also presumably affect the sperm ratio, and thus the chromosome present at conception. These theories include altering the mother-to-be’s vaginal pH, altering the father’s sexual activity, and only trying to conceive on particular days of the woman’s cycle.
Overall, the chance of determining a baby’s gender is uncertain, but it would have to affect the sperm’s contribution prior to conception, when the genotype is determined.
Gender is based on the sex chromosomes, which are contributed by the egg and sperm. So a baby's gender is already present at conception - everything else is how the genetics play out.