In humans, pregnancy is possible when sexual intercourse occurs at the same time as the female is ovulating. After ovulation, the egg lives for 12 to 24 hours. The egg must be fertilized in that time for a woman to become pregnant.

Human sperm will remain fertile for 1–2 days. When the sperm arrives in the acidic conditions of the vagina, they swim forward towards the uterus and the fallopian tube, where fertilization takes place.

Out of the strongest and fastest sperm, only one sperm cell penetrates the surface of the ovum. Fertilization occurs when the nuclei of a sperm cell and an egg cell fuse together. When this happens, the two gametes form a diploid cell, known as a zygote. The ovarian membrane becomes impermeable to sperm immediately after fertilization.

Fertilization:​ 1. The sperm cell. 2–3. The acrosome reaction makes it possible for the sperm cell to penetrate the zona pellucida. The cell membranes of the sperm cell and the egg cell are combined. 4. The nucleus of the sperm cell moves inside the egg.