One to two hours after intercourse, the sperm have swum 30-40 cm up the fallopian tube and reach the egg (ovum). Approximately 20 hours after intercourse, the first cell division of the fertilised egg takes place. Five days later, the fertilised egg attaches to the lining of the uterus and continues to grow. The egg measures 140 μm in diameter and weighs 1.65 μg.