Reproductive Endocrinology and Infertility
Infertility - Fertility And The Normal Menstrual Cycle

The normal menstrual cycle is the result of a complex interaction between multiple ovarian and pituitary hormones whose function is to produce an oocyte (egg) that can be fertilized and implant in the woman’s uterus. In the absence of fertilization and implantation the endometrium (lining of the uterus) is shed, resulting in menstrual bleeding. The process then repeats itself in the next cycle, again preparing the woman for possible pregnancy. A menstrual cycle is defined as that interval of time from the start of one period until the start of the next period. The normal menstrual cycle is 28 + 7 days. By convention, cycle day 1 is designated as the first day of full bleeding. Premenstrual spotting, which occasionally occurs, is not counted as the first cycle day. A menstrual cycle less than 21 days or greater than 35 days is abnormal and could represent an underlying problem. The normal duration of menstrual bleeding is 7 days or less; bleeding longer than 7 days is abnormal.

A cohort of follicles (eggs) is recruited during the first week of the menstrual cycle. Although a large number of follicles are initially recruited and start growing, only one of them will continue to develop and ovulate. This one is termed the "dominant follicle". The dominant follicle is usually selected by the 5th or 6th day of the cycle. During the next week the dominant follicle will continue to grow while the other follicles that were initially recruited, but were not selected, will regress and die. The primary hormone made by the developing dominant follicle is estradiol. It is the rise in estradiol from the dominant follicle that is responsible for the cessation of menstrual bleeding. Estrogen acts to regenerate the endometrium that was shed at the beginning of the cycle. As the estrogen level in the blood continues to rise, it will stimulate the release of another hormone from the pituitary gland called luteinizing hormone or LH. The pituitary is a small gland deep in the brain that controls the function of multiple endocrine glands including the ovaries. This surge in LH, which generally occurs around cycle day 13, will cause the dominant follicle to release its egg and ovulate. The egg is then picked up by the fallopian tube where it awaits the arrival of sperm. The egg can survive for only about 12 –24 hours. Therefore, if sperm do not arrive and fertilize the egg within that time interval, pregnancy will not occur that cycle.

The peak time for fertility is on the day of and the day after the LH surge. The LH surge can be easily monitored in the urine using a commercially available ovulation detection kit. These kits are available without a prescription and can be found in most pharmacies. Therefore if one was able to have intercourse only once during the cycle, it should occur on one of the aforementioned days in order to maximize the chance for pregnancy. Although urine LH monitoring is helpful in determining whether or not a woman is ovulating, there is no scientific evidence that timing of intercourse based on detection of the LH surge improves pregnancy rates. Intercourse every day or every other day at mid-cycle will maximize the chance for conception that cycle. Sperm can survive in the female reproductive tract for several days and pregnancy can occur when a single episode of intercourse occurs up to five days before ovulation. However, when intercourse occurs one day or more after ovulation, pregnancy is extremely unlikely that cycle. Therefore, it is better to err on the side of having intercourse before ovulation rather than after. Once a positive urine LH surge has been detected there is no benefit to repeated testing in subsequent cycles to time intercourse. This can become expensive and as long as the menstrual cycle length does not significantly change, ovulation can be presumed to be occurring in subsequent cycles.

Human reproduction is inefficient. Once fertilization has occurred, it is estimated that over 60% of these early embryos fail to implant normally and produce an ongoing pregnancy. Under optimal conditions, the chance of conception during one ovulatory cycle is only about 25%. Of those women that do conceive, 15-20% will miscarry the pregnancy, usually in the 1st trimester. The chance of conception can be significantly lowered by other factors such as advanced age of the female, endometriosis, low sperm count, etc. This "baseline fertility rate" should be kept in mind when interpreting success rates for the various infertility treatments.