With in vitro fertilization (IVF), a woman's eggs are fertilized outside of the body in the IVF lab. Here is how IVF fertilization works.

Eggs

Test tubes containing the eggs that were retrieved are transferred to the fertility clinic's laboratory. Technicians use a microscope to help them separate the individual eggs. After being bathed in a medium that removes any impurities, the eggs are moved to a carbon-dioxide incubator in separate Petri dishes. Fertilization may take place anywhere between two and six hours after the retrieval process, because some eggs may need more time than others to mature.

Sperm

The sperm collected at the clinic are also transferred to the laboratory to be prepared for insemination. A high concentration of sperm is placed around each egg in dishes with another type of growth medium. They are left together overnight (in an incubator) to allow “natural” fertilization to take place.

Embryo Development

These dishes go back into the incubator and an embryologist starts monitoring their development 18 hours after fertilization. Although the embryo is at the single cell stage, the specialist looks for two clear bubbles (pronuclei), which indicate that the embryo contains both the male and female genetic material. Embryos without two pronuclei are not viable.

During the next 24 hours, viable embryos divide into two to four cells. At this stage, an embryo may be transferred to a woman’s uterus. However, sometimes it is preferable to incubate embryos for three to five days.

At three days, the embryo is usually at the six or eight-cell stage. At this point, the embryologist can determine additional factors that affect viability. By five days, the embryo has divided into 12 or 16 cells and is called a blastocyst. Using a blastocyst is more likely to result in a successful uterine implantation for women with prior miscarriages or unsuccessful IVF attempts.

ICSI and IVF

In some instances, a single sperm is injected into a female egg prior to IVF to increase the odds of fertilization. This procedure is know as intracytoplasmic sperm injection, and may be used in cases of male factor infertility, when past IVF cycles have not resulted in fertilized eggs, or to increase chances of fertilization when few eggs have been retrieved.