Is just a drop of semen enough to lead to pregnancy?
Yes, only one sperm is neede to fertilize the egg! But because so many sperm die or don't make it from the vagina as far as the egg (which is normally fertilized in the fallopian tube), less sperm means a lower chance of pregnancy and more sperm means a higher chance.
The moment of conception Normally, only one sperm can break through the outer membrane of the egg; once the sperm enters the egg, the sperm loses its tail.
Implantation After six days, the cluster of cells develops a hollow cavity. Called a blastocyst, it attaches itself to the wall of the womb. Some cells grow into the womb''''s lining and the fetal life-support system (the placenta) begins to form.