The sex of a baby is determined at the moment of conception, according to WebMD. If the sperm has an X chromosome, the baby is a girl, but if it has a Y chromosome, the baby is a boy. However, the fetus doesn't develop reproductive organs for another 60 days.
For the first several weeks after conception, the baby follows the same blueprint for development, regardless of whether it is male or female. It is only after the male fetus starts to produce testosterone that the sex characteristics begin to develop. The hormone begins to change the genetic activity of the cells that form the reproductive organs. By this time in development, the male fetus has fully developed mammary papillae, along with the same blood vessels and nerves as females. However, scientists have found no evidence that men ever nursed their young, meaning they are likely a feature that natural selection did not remove.
The moment the sperm meets the egg also determines the genetic characteristics of the child. The fertilized egg is a single cell, but that cell begins dividing rapidly, even before it leaves the fallopian tube. It slowly travels through the fallopian tube and then attaches to the uterus. It remains there, until it is ready for birth.