In order for a natural pregnancy to occur, the woman's ovaries must be able to expel a mature egg that can be fertilized by a sperm. For his part, the man must have an adequate sperm production to be able to expel a good number of motile spermatozoa in the ejaculate. On the other hand, sexual intercourse must coincide with the moment of ovulation so that both gametes (egg and sperm) can meet in the fallopian tubes and fertilization can take place.
Although it may not seem like it, the process of reproduction is very complex, so there are couples who are not able to achieve it on their own, since they suffer from some fertility problem. Assisted reproduction techniques are intended to solve these problems and help these couples to achieve pregnancy. For example, there are medications that induce ovulation in women with irregular and anovulatory menstrual cycles, artificial insemination (AI) brings the sperm closer to the woman's uterus to facilitate the egg-sperm encounter, intracytoplasmic sperm injection (ICSI) achieves fertilization, etc.
In short, the greater the infertility problem, the more complex the assisted reproduction technique will be needed for gestation to take place.