Pregnancy can occur only if the egg encounters sperm within 24 hours of ovulation. After ejaculation, sperm swim from the vagina up through the cervix into the uterus. From the uterus, they make their ascent into both of the fallopian tubes, propelled by their tails and by the contractions of the uterine walls. If ovulation has not occurred when they arrive at the fallopian tubes, they can survive up to three days waiting to encounter and fertilize an egg.
When egg and sperm interact, many sperm attempt to burrow through the outer membrane of the egg called the zona pellucida (see Figure 1.4). When the first sperm is successful, fertilization occurs. The sperm fuses with the egg, transferring its genetic material. The egg then becomes impenetrable to other sperm.
Figure 1.4 Fertilization: Sperm burrowing
through the zona pellucida.
The fertilized egg, now known as an embryo, travels to the uterus and cell division occurs over the course of four days. On day five, the embryo rearranges itself into a hollow ball of cells called a blastocyst. Pregnancy may occurs once the blastocyst or embryo has attached to the endometrium (see Figure 1.5).
Figure 1.5 Pregnancy: Embryo implantation.