Before a man’s sperm can fertilize a woman’s egg, the head of the sperm must attach to the outside of the egg. Once attached, the sperm pushes through the outer layer to the inside of the egg (cytoplasm), where fertilization takes place.
Sometimes the sperm cannot penetrate the outer layer, for a variety of reasons.
The egg’s outer layer may be thick or hard to penetrate or the sperm may be unable to swim. In these cases, a procedure called intracytoplasmic sperm injection (ICSI) can be done along with in vitro fertilization (IVF) to help fertilize the egg. During ICSI, a single sperm is injected directly into the cytoplasm of the egg.
There are two ways that an egg may be fertilized by IVF: traditional and ICSI. In traditional IVF, 50,000 or more swimming sperm are placed next to the egg in a laboratory dish. Fertilization occurs when one of the sperm enters into the cytoplasm of the egg. In the ICSI process, a tiny needle, called a micropipette, is used to inject a single sperm into the center of the egg. With either traditional IVF or ICSI, once fertilization occurs, the fertilized egg (now called an embryo) grows in a laboratory for 1 to 5 days before it is transferred to the woman’s uterus (womb).