At the beginning of this week, your conception starts. Sperm has found the egg in the uterine tube – the miracle of life has begun, in a process that takes about 24 hours. The most potent sperm cells survive up to 5 days inside the female genitalia, but the median survival rate is usually 48 to 72 hours after ejaculation.

Although many sperm cells penetrate the outer lining of the egg, only one will be able to fertilize it in a trajectory that lasts about 20 minutes. At the center of the egg, your chromosomes and the father’s will merge, and millions of little pieces of genetic information will come together to form the baby’s chromosomes.

The nucleus of the sperm and the egg fuse together. When that happens, it marks the development of the zygote – the cell, which will eventually form the embryo and end the process of fertilization. After about 18 hours of conception, it is possible to notice, through a microscope, the female and male nuclei. At this moment, your partner’s X or Y chromosome defines the biological sex of the baby. Yes, you read that right: the father is always the decisive factor in what the child’s biological sex will be. For a long time, throughout history, the blame has unjustly been placed on women for not giving birth to a male son, the most desired outcome of pregnancy for previous generations.

Sometime between the end of this week and the beginning of the next, an important process begins: the embryo attaches itself to the uterine lining, thus becoming a blastocyst. During the implantation of the blastocyst in the uterus, there may be some slight vaginal bleeding, which most women won’t even notice. If this happens to you, don’t worry: just go to your doctor to address any questions or concerns you might have.