Infertility can result from a variety of causes, which can affect either the female or the male partner. Many cases are due to anatomical defects in the Fallopian tubes, through which oocytes must travel from the ovary to the uterus. Some women are infertile (or subfertile) because of hormonal disorders that result in the failure to produce normal metaphase II eggs. Other cases of infertility or subfertility result from a failure of the male partner to produce or ejaculate adequate numbers of normal sperm. In vitro fertilization (IVF), which was originally developed as a treatment for tubal defects, is now widely used in the treatment of both male and female reproductive disorders. The basic procedure of in vitro fertilization is to recover metaphase II eggs from the ovary and culture them together with sperm. Fertilization is assessed 12 to 18 hours later by microscopic observation to detect the formation of two pronuclei. Fertilized eggs can then be returned to the Fallopian tube or uterus. Alternatively, they can be maintained in culture for a few additional days, during wich the early embryonic cell divisions take place. In vitro fertilization was initially developed to bypass defects in the Fallopian tube, which present an obvious anatomical block to a natural pregnancy. IVF is also effective for other reproductive disorders. For example, hormonal defects resulting in the failure to produce metaphase II eggs can be treated by administration of appropriate hormones to the female partner. This treatment results in the production of multiple eggs, which can then be fertilized in vitro, followed by the return of successfully fertilized eggs to the uterus.