Sophisticated methods of enhancing fertility and a new drug that prevents pregnancy are among the most important recent developments in the field of human reproduction.

Last month, federal officials gave final approval to Norplant, a contraceptive that offers continuous protection against pregnancy for up to five years. The drug, which uses the same synthetic hormones as the birth control pill, is implanted by a physician through a small incision underneath the skin of the upper arm. It is expected to be commercially available next month.

Considered safer, more effective and less expensive than the pill, Norplant represents the first major advance in contraception in the past 30 years. Proponents predict it will reduce the number of unwanted pregnancies, particularly among teenagers.

At the same time, scientists are experimenting with ways to extend the traditional limits of fertility, a trend sociologists say has been fueled by the number of aging baby boomers who are approaching, or past, 40 and have delayed marriage and childbearing.

Last fall, California researchers announced that they had successfully induced five of seven post-menopausal women to carry and bear children. Their success raises the possibility of treating more women over 40 for infertility.

The key to this advance is egg donation, a new procedure that doctors hope will help a variety of infertile women, including those who experience early menopause or who do not respond to ovary-stimulating drugs.

At nearly 50 medical centers around the country young, fertile, healthy women are donating or selling their eggs, which are fertilized in a petri dish by sperm from the husband of an infertile woman. These pre-embryos are then implanted in an infertile woman who, it is hoped, will deliver a healthy baby, albeit one that is not genetically related to her.

A small group of infertile women have undergone transcervical angioplasty, a new non-surgical procedure for opening blocked fallopian tubes. The technique, a spinoff from the procedure used by cardiologists to unclog blocked heart arteries, involves threading a tiny tube called a catheter through the uterus and into the blocked fallopian tubes. By inflating the balloon, the tubes are unblocked, enabling sperm and egg to meet and fertilization to occur.

A study published several months ago in the Journal of the American Medical Association reported that tubes were unblocked in 64 of 77 women who underwent this procedure; 22 of them became pregnant within three months.

Medical experts say that men also are seeking help with fertility problems, such as inactive sperm that are incapable of penetrating eggs. One of the newest techniques is micromanipulation, which allows physicians to implant sperm directly into an egg. The procedure is of limited use because it must be done in conjunction with in vitro fertilization, an expensive and frequently unsuccessful procedure. Experts estimate that fewer than half of infertile men can be helped by this method.