To become pregnant, the following steps must occur:
Sperm transport — The sperm must be deposited and transported to the site of fertilization.
Egg transport — Ovulation must occur and the egg must be “picked up” by the tube.
Fertilization and embryo development — Union between the sperm and egg must result.
Implantation — The embryo must implant and begin to grow in the uterus.
Once the embryo reaches the blastocyst stage, approximately five to six days after fertilization, it hatches out of its zona pellucida and begins the process of implantation in the uterus.
In nature, 50 percent of all fertilized eggs are lost before a woman’s missed menses. In the in vitro fertilization (IVF) process as well, an embryo may begin to develop but not make it to the blastocyst stage — the first stage at which those cells destined to become the fetus separate from those that will become the placenta. The blastocyst may implant but not grow, or the blastocyst may grow but stop developing before the two week time at which a pregnancy can be detected. The receptivity of the uterus and the health of the embryo are important for the implantation process.