Ovulation

During the average woman's menstrual cycle there are six days when intercourse can result in pregnancy; this “fertile window” comprises the five days before ovulation and the day of ovulation itself. Just as the day of ovulation varies from cycle to cycle so does the timing of the six fertile days. Reliable methods to predict ovulation are lacking, therefore predicting the fertile window is also unreliable. Clinical guidelines suggest the cycle days during which the fertile window is most likely to occur, but these guidelines are outdated.

In the research where participants were 221 healthy women who were planning a pregnancy, the fertile window occurred during a broad range of days in the menstrual cycle. On every day between days 6 and 21, women had at minimum a 10% probability of being in their fertile window. Women cannot predict a sporadic late ovulation; 4-6% of women whose cycles had not yet resumed were potentially fertile in the fifth week of their cycle.

In only about 30% of women is the fertile window entirely within the days of the menstrual cycle identified by clinical guidelines—that is, between days 10 and 17. Most women reach their fertile window earlier and others much later. Women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular.

On research participants ovulation occurred as early as the eighth day and as late as the 60th day of the menstrual cycle.. Overall, an estimated 2% of women were in their fertile window by the fourth day of their cycle and 17% by the seventh day (based on 213 women). This percentage peaked on days 12 and 13, when 54% of women were in their fertile window. If ovulation was delayed, women reached their fertile days much later. Among women who reached the fifth week of their cycle, 4-6% were in their fertile window.

The precision of the estimates can be improved by using women's reports of the usual length of their cycle. The women reported usual cycle lengths as short as 19 days and as long as 60 days, with 28 days being the most common. Self reported cycle length can be useful in predicting whether a woman is in her fertile window.

Intercourse during the fertile window is not sufficient to produce pregnancy. Pregnancy depends on the viability of the sperm and egg, the receptivity of the uterus, and other factors that vary widely among couples. Within the six fertile days of each cycle, the probability of conception is lowest on the first day. This is most relevant for the earliest days in the menstrual cycle, which are also the most likely to be the earliest (and least fertile) day of the fertile window.

Current clinical guidelines about a woman's potentially fertile days have been based on two assumptions—that ovulation occurs 14 days before the next menses and that women are fertile for several days before and after ovulation. It follows that in the usual menstrual cycle lasting 28 days, the fertile days would fall between days 10 and 17. The assumptions are, however, outdated. Firstly, only a small percentage of women ovulate exactly 14 days before the onset of menses. This is true even for women whose cycles are usually 28 days long. Among the 69 cycles for 28 days in the study, ovulation occurred 14 days before the next menses in only 10%. Time from ovulation to next menses ranged from 7 to 19 days (days 10 to 22 of the menstrual cycle). Thus, the fertile window can occur much earlier or later in the cycle than clinical guidelines suggest. On average, at least 10% of women with regular cycles were in their fertile window on any given day of their cycle between days 6 and 21. The timing of the fertile window is even less predictable for women with less regular cycles, which includes adolescents and women in their perimenopause.

Regarding the second assumption, the evidence for fertile days after ovulation comes from studies using crude measures of ovulation (for example, basal body temperature). With more precise measures, the fertile window does not seem to extend beyond the day of ovulation. It follows that women reach their fertile days earlier in the cycle than suggested by current guidelines. For example, women with regular 28 day cycles are most likely to be potentially fertile on days 8-15 of their menstrual cycle.

Any couple wishing to have a baby can easily avoid the uncertainty of predicting fertile days by engaging in intercourse two or three times a week.

Abstinence on specific days of the menstrual cycle remains a method of family planning for many couples worldwide. Women should be aware that no calendar method is completely effective. Our data suggest there are few days in the menstrual cycle during which some women are not potentially capable of becoming pregnant—including even the cycle day on which they may expect their next menses to begin.

[1] Allen J Wilcox, David Dunson, Donna Day Baird: The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study, ncbi.nlm.nih.gov/, extracted