Inside you DNA is a "code". If there is code, you have to have a code maker. The code is what identifies us a human and one of two genders. When the egg of the woman is fertilized by the sperm of the man, the egg splits into two identical eggs which them splits into 4 identical eggs. This process continues until something tells one cell to start forming bones, another a brain, another to forma heart. This is either built into this code or an outside source does it. God says He formed us from the womb so I am believing the He is controlling the process.