Ovum, sperm, hormone cycles, fertility windows … if you need a refresher on the facts of life, you’ve come to the right place. OB-GYN and reproductive specialist Deborah Metzger, MD, offers a clear review of the basic facts and new research that will enlighten your baby-making efforts!
We live in a culture where sex is everywhere: talk shows, magazine articles, advertising, and movies—just to name a few. But when it comes to sex for reproduction, not many people appreciate how the sperm and egg come together and how very delicate and sensitive the process is.
Ovaries and Eggs
Every woman is born with immature eggs in her ovaries. Just before birth, the number of eggs in the baby girl’s ovaries is roughly a few million, but by the time puberty begins, this number has diminished to about 400,000. Of these, only three hundred or so will ever be released. As you age, the number of eggs in your ovaries continues to diminish until the ovary runs out of eggs and you enter menopause (somewhere between 45 and 55 years old, usually).
The development and release of the egg depend on a delicate balance of hormones, chemicals that signal the body’s organs to do particular jobs. Some of these hormones are produced in the ovaries. Others come from two glands in the brain, the hypothalamus and the pituitary.
The Egg’s Journey
The process of preparing an egg for release begins the first day of menstruation. During menses, the level of the hormone estradiol is at its lowest and this causes the hypothalamus and pituitary to release follicle stimulating hormone (FSH) and luteinizing hormone (LH). These two hormones act in concert to recruit about 20 eggs and to stimulate the ovary to produce estradiol. Estradiol encourages the growth of the lining of the uterus so that it becomes thick and lush.
Although approximately 20 eggs may start this process each month, only one egg usually survives. The mature egg(s) is released in response to a sudden release of a large amount of LH (this is what is detected with ovulation predictor kits), which occurs approximately 14 days after the beginning of a menstrual period. The LH surge signals the ovary to begin producing progesterone, which will prepare the lining of the uterus for implantation of a fertilized egg. Once released from the ovary, the egg begins its journey by entering the fallopian tube where it may encounter eagerly awaiting sperm and become fertilized.
The fertilized egg continues making its way through the fallopian tube during which time it begins to divide, resulting in the formation of a blastocyst. Six to 10 days (cycle day 20 to 24) after ovulation/fertilization, the blastocyst arrives in the uterine cavity and implants in the endometrium, or uterine lining. If implantation occurs, the blastocyst produces human chorionic gonadotropin (hCG), which prolongs the secretion of estrogen and progesterone from the ovary until the placenta is developed sufficiently to take over this function. If fertilization or implantation fails to take place, the ovary “automatically” stops production of estrogen and progesterone, causing the prepared endometrium to be cast off as menstrual flow.
The eggs in the ovary are vulnerable to tobacco, chemicals, and radiation. Once eggs are damaged or destroyed, they are not replaced. The delicate balance between the hypothalamus and pituitary is vulnerable to stress, obesity, diet, exercise, and many other factors that can be managed through preconception preparation.
Testosterone and Sperm
In men, fertility means the ability to make a woman pregnant. To do this, the man’s reproductive system needs to produce and store sperm. It also needs to transport sperm outside of his body, so it can enter the woman’s reproductive tract.
There are several aspects of the male reproductive system that are analogous to the female reproductive system. Testosterone and sperm production are regulated by LH and FSH produced by the pituitary gland. Unlike the female reproductive system, there is no cyclicity and the male continually produces millions of sperm until death.
The Sperm’s Journey
The entire process of spermatogenesis (production and maturation of the sperm) takes place in the testicle, where conditions are generally optimal for sperm production: a temperature 2° centigrade lower than the core body temperature and a barrier between the blood and the testis (to prevent the production of sperm antibodies). The sperm move gradually from the testis to the epididymis, an organ that stores and nourishes them as they mature. Once sperm are completely mature, they move into the vas deferens. This tubal structure connects the epididymis with the seminal vesicles, the two pouch-like glands that provide storage for the mature sperm.
The entire process of spermatogenesis from cell division to ejaculation takes about 84 days. Just prior to ejaculation, sperm from the seminal vesicles combine with a thick fluid from the prostate gland to create the semen (a mixture of secretions and sperm), which may then be deposited in the vagina.
The earliest stages of spermatogenesis are easily influenced by fever, prolonged exposure to heat, and chemical exposure. Thus, suboptimal sperm will be ejaculated for two to three months following exposure to these conditions. Likewise, any fertility treatment directed toward the male may take three to four months to become apparent in the semen analysis. Sperm counts drop with frequent ejaculation and may rise with two to seven days of abstinence. Longer periods between ejaculations may result in a drop in sperm quality. Generally, these variations in the quality of semen have no demonstrable effect on the fertile population, but they may contribute to infertility in those having trouble conceiving.
Understanding the fine tuning of the reproductive machine can help you make it work more efficiently for you. Or at the very least it’ll help you appreciate the miracle of a healthy pregnancy even more!