Science of Fertility
Professor Emeritus James B. Brown
Ovarian Activity and Fertility
Ovulation – the release of an ovum by the ovary and therefore the only time during the cycle when the ovum is exposed for fertilization –is the central event of the fertile ovarian cycle. It determines the time when pregnancy can occur from an act of intercourse which is the period of 3-4 days (rarely 5-6 days depending on the cervical mucus) before ovulation determined by the fertilizing life span of the sperm and up to 24 hours after ovulation determined by the fertilizable life span of the ovum. Outside this time period a woman cannot conceive from an act of intercourse no matter how hard she tries. Even within this time period, pregnancy from an act of intercourse is not a certainty, the chances vary depending on the couple and the
timing of intercourse in relation to ovulation. Maximum fertility is reached during the period of 24 hours before ovulation and several hours afterwards. If the chances of pregnancy at this time are 70% per cycle, it takes two cycles for 90% of couples having intercourse on the most fertile day to achieve pregnancy. If the chances at the beginning of the fertile period are 10% per cycle, it takes 24 cycles for 90% of couples having intercourse at this time to achieve pregnancy. Many authorities would say that the chances are much less than the figures given. Even with in vitro fertilization (IVF), which many would like to think is the ultimate in assisted pregnancy, most health funds allow up to six cycles of treatment. Thus, couples who do not follow the rules of the Billings Ovulation Method® (BOM) and do not become pregnant should not conclude that the rules do not apply to them, chance has been on their side. Alternatively, if they have had intercourse on the most fertile day they should not expect pregnancy to follow as a certainty. In animals, Nature has ensured a maximum fertilization rate (but not 100%) by restricting intercourse to the most fertile day of their cycle by the phenomenon of oestrus. Thus, the assessment of ovarian activity and the accurate timing of ovulation are basic requirements in natural family planning (NFP) for avoiding pregnancy and under all circumstances including IVF for achieving it.
Physiology of Ovulation
Ovulation occurs over a period of about 15 minutes and even when more than one ovum is released, as in a twin pregnancy, the multiple ovulations occur very close together in time. Thus, ovulation is the one event in the menstrual cycle with a very precise time frame. The ovulatory mechanism produces the two ovarian hormones, oestradiol and progesterone. The ovum is contained within an ovarian follicle and matures as the follicle goes through its rapid growth phase. During this rapid growth phase, the follicle produces increasing amounts of oestradiol. This oestradiol stimulates the glands of the cervix to secrete a particular type of mucus (“mucus with fertile characteristics”) which is essential for the sperm to pass through the cervix to reach the ovum. Oestradiol also stimulates growth of the endometrium which lines the body of the uterus, i.e., the womb (“proliferative phase”). After rupture of the follicle and release of the ovum, both progesterone and oestradiol are secreted by the corpus luteum which forms from the ruptured follicle. The rapid rise in progesterone secretion strongly
counteracts the effect of oestrogen on the cervix and vaginal epithelium and thus causes the progesterone change (PC) in the mucus pattern which occurs near ovulation and defines the Peak day (the last day of mucus with fertile characteristics before the change). Progesterone also acts on the oestrogen-primed endometrium making it suitable for implantation of the fertilized ovum (“secretory phase”). In the absence of pregnancy, secretion of oestradiol and progesterone reaches a maximum approximately 7 days after ovulation and then declines. This leads to shedding of the endometrium as menstrual bleeding 11-16 days after ovulation. The cyclical changes in ovarian activity are controlled by the secretion of two hormones by the pituitary gland situated in the brain, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Production of these two hormones is controlled in turn by an area of the brain called the hypothalamus. The hypothalamus acts as a computer, analysing nervous signals from other areas of the brain including those generated by the emotions and by environmental factors, such as stress and nutrition; it also analyses hormonal signals (oestradiol and progesterone) generated by the ovaries and other endocrine glands and
transmitted by the blood stream. The sum total of these effects determines the quality of the ovarian activity produced.