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The Fertile Window - Scientific Literature Review

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The Fertile Window - Scientific Literature Review

Intercourse timing and frequency relative to ovulation is possibly the most important factor affecting the chances of conception. It is also the element over which couples trying to conceive have the most degree of control.

Couples who are aware of their fertile time and focus intercourse during this time have been found to have much higher conception rates than couples who are unaware of their most fertile time (Hilgers et al 1992). Identifying the fertile window is thus of great importance for couples trying to conceive.

Identifying the fertile window also allows couples and their healthcare providers to retrospectively determine if they have been timing intercourse optimally and may help to decide whether diagnostic investigations or interventions are needed (if intercourse has been optimally timed over several cycles with no pregnancy) or whether they may be postponed (if intercourse has not been timed optimally).

What is the fertile window?

The fertile window is made up of the days in the menstrual cycle where pregnancy is possible. The length of the fertile window has been determined by documenting the farthest away from ovulation that intercourse can result in conception. To determine the length of the fertile window and the probability of pregnancy relative to ovulation we need to know the maximum lifespan of the male sperm in the female reproductive tract. While the human ovum has a lifespan that is likely not more than 24 hours, making conception highly unlikely after the day of ovulation, the lifespan of the sperm is potentially longer and more variable.

The studies

A number of studies have assessed the length of the fertile window and the probability of pregnancy on different days of the menstrual cycle relative to ovulation. To determine this fertile window and day-specific pregnancy probabilities, investigators of several studies asked couples to record each act of sexual intercourse during the menstrual cycle. Ovulation was determined using a variety of methods. The majority of these studies estimated ovulation using basal body temperature (Barrett and Marshall 1969; Royston 1982; Schwartz et al 1980; Columbo and Masarotto 2000), yet studies have also been done using cervical fluid (World Health Organization 1983; Hilgers et al 1992) or the measurement of urinary hormone levels (Wilcox et al 1995) as the marker of ovulation.

Findings

Based on two widely cited studies (details below) the maximum fertile window has been determined to include the day of ovulation and the five days before ovulation. The Barrett and Marshall (1969) study was based on data from 241 British married couples who were charting basal body temperatures. The Wilcox et al (1995) study relied on data from 221 healthy North Carolina women who were planning to become pregnant.

The most likely day for intercourse to result in conception according to the 1969 study was two days before ovulation. According to the 1995 study, the most likely day for intercourse to result in pregnancy was on the day of ovulation itself. Further studies have been done, however, to account for measurement error in both studies and to exclude early pregnancy losses from the Wilcox et al data set. With the corrected data, the most probable days for intercourse to result in pregnancy have been found to be one or two days before ovulation rather than the day of ovulation itself (Stanford et al 2002).

The limited sample size of the studies, however, must be noted, though a larger multi-center study from 881 women including 7017 menstrual cycles (Colombo and Masarotto 2000) also suggests that intercourse on the day prior to ovulation is most likely to result in pregnancy.

While the results from the studies may be based on a small sample size and may be potentially error prone, they show that intercourse on the few days before ovulation and the day of ovulation itself are the days most likely to result in conception. This has been confirmed by a much larger study involving 119,398 cycles from women trying to conceive which revealed that 94% of all conception cycles included intercourse on at least one of the three days up to and including the day of ovulation (FertilityFriend.com 2004). Additionally, all of these studies found that multiple acts of intercourse within the fertile window increased the probability of conception.

Conclusions and recommendations for those trying to conceive:

The combined results of these studies indicate the following:

  1. Couples trying to conceive should try to focus intercourse within the days immediately preceding and including ovulation.

  2. Intercourse in the fertile time should not be limited unless there is a medical reason to do so.

  3. Diagnostic investigations or treatment interventions might be premature if there is prolonged unwanted non-conception yet intercourse has not been focused on the most fertile days.

  4. Couples who are hoping for pregnancy should not anticipate positive results or begin to test for pregnancy when intercourse has not occurred on at least one of the most fertile days.

  5. Additionally, because the most fertile time begins at least a couple of days before ovulation, couples using devices such as OPKs (ovulation prediction kits) or fertility monitors should begin to have intercourse prior to observing a positive or a peak so as not to miss an opportunity to conceive (since these devices typically only indicate a peak or positive on the day prior to ovulation).


The results of these studies also highlight that learning to recognize the most fertile days which occur prior to ovulation is of prime importance for couples trying to conceive.


References

Barrett, J., Marshall, John (1969). "The Risk of Conception on Different Days of the Menstrual Cycle." Population Studies 23(3): 455-461.

Columbo, B., Masarotto, G. (2000). "Daily Fecundability: First Results from a New Data Base." Demographic Research 3(5).

FertilityFriend.com (2004) Intercourse Timing and Frequency (http://www.fertilityfriend.com/Faqs/Intercourse_Timing_and_Frequency.html).

Hilgers, T. W., K. D. Daly, et al. (1992). "Cumulative pregnancy rates in patients with apparently normal fertility and fertility-focused intercourse." J Reprod Med 37(10): 864-6.

Royston, J. P. (1982). "Basal body temperature, ovulation and the risk of conception, with special reference to the lifetimes of sperm and egg." Biometrics 38(2): 397-406.

Schwartz, D., Macdonald, P.D.M., Heuchel, V. (1980). "Fecundability, Coital Frequency and the Viability of the Ova." Population Studies 34(2): 397-400.

Stanford, J. B., G. L. White, et al. (2002). "Timing intercourse to achieve pregnancy: current evidence." Obstet Gynecol 100(6): 1333-41.

Wilcox, A. J., C. R. Weinberg, et al. (1995). "Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby." N Engl J Med 333(23): 1517-21.

World Health Organization (1983). "A prospective multicentre trial of the ovulation method of natural family planning. III. Characteristics of the menstrual cycle and of the fertile phase." Fertil Steril 40(6): 773-8.

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