Study Suggests That Peak Mucus Day Has Highest Probability for Conception
Natural Family Planning

Richard J. Fehring
Reprint from Current Medical Research
Vol 15, No 1-2, Winter/Spring 2004
Washington, DC
Reproduced with Permission

Researchers at the University of Utah and the National Institute of Environmental Health Sciences recently conducted a study to determine the day-specific probabilities of pregnancy in relation to the timing of intercourse and the quality of self-observed vulvar mucus among a group of normal and sub-fertile couples.1 The couples came from six different Creighton Model FertilityCare™ System centers in four different cities in Missouri, Nebraska, Kansas, and California. The Creighton Model (CrMS) is a cervical mucus system of natural family planning that utilizes a standardized vulvar discharge recording system. The researchers retrospectively obtained two groups of CrMS system users; one group was comprised of couples with normal fertility who had the initial intention of avoiding pregnancy and the other group were couples defined as sub-fertile based on having previous difficulty in achieving pregnancy. There were 309 couples in the normal fertility group that generated 1,681 usable data charts and 117 couples in the sub-fertile group that yielded 373 cycles of usable data. Two research assistants and one researcher identified the peak day for each cycle and the day or days of intercourse 6 days before and 4 days after the peak day of cervical mucus. A numerical score for the peak day and the 5 days before was calculated based on a mucus scoring system developed by Dr. Thomas Hilgers from the Pope Paul VI Reproductive Institute (Omaha). A statistical model was used to determine the day specific probabilities of pregnancy and to accommodate multiple acts of intercourse during the potential fertile phase.

The researchers found 81 pregnancies among the 1,681 cycles from the couples of normal fertility and 30 pregnancies among the 373 cycles of data from the couples with sub-fertility. All pregnancy cycles had acts of intercourse 6 days before to 3 days after the peak day of cervical mucus. The researchers calculated that the highest probability of conception was on the peak day of cervical mucus with a probability of 0.38 for couples of normal fertility and 0.14 for sub-fertile couples. The probability of pregnancy was higher than 0.05 from 3 days before to 2 days after the peak day of cervical mucus for couples of normal fertility and from 1 day before to 1 day after for couples of sub-fertility. They also found a linear positive relationship between the mucus cycle scores and the probability for pregnancy among the normal fertile couples but not the sub-fertile couples. However, there was no significant difference between the mucus cycle scores of the normal and sub-fertile groups. The researchers concluded that a standardized system of rating vaginal mucus discharges can help couples of normal fertility and of sub-fertility identify the days of the menstrual cycle that have the greatest likelihood of conception. They also speculated that the highest probability of pregnancy occurred on the peak mucus day - their estimated day of ovulation.


I agree with the analysis, that the peak day of cervical mucus coincides with the day of the highest probability of pregnancy. However, I would also say that the highest probability of pregnancy is on the two days before the day of ovulation and that the highest quality of cervical vaginal mucus is usually on those two days. I base this statement on the research by Dunson and others that indicated the highest probability of pregnancy is on the day before ovulation and on my research that showed the highest average of quality mucus on the two days before the estimated day of ovulation (based on the day after the urinary LH surge).2,3

The authors do indicate a limitation of this study: no reference marker was given for the day of ovulation, e.g., follicular ultrasound. Another limitation of the study is that the subfertile couples were on average older then normal fertile couples. For example the average age of the normal fertile women was 27 years and the sub-fertile 30.3 years.

This affected the difference in probability of pregnancy between the normal fertile and the sub-fertile groups of couples. (RJF)


1 Stanford, J.B., Smith, K.R., & Dunson, D.B. Vulvar mucus observations and the probability of pregnancy. Obstetrics & Gynecology. 2003;101:1285-1292. [Back]

2 Dunson, D.B., Baird, D.D., & Wilcox, A.J., et al. Day-specific probabilities of clinical pregnancy based on two studies with imperfect measures of ovulation. Human Reproduction 14 July 1999):1835-1839. [Back]

3 Fehring, R. Accuracy of the peak day of cervical mucus as a biological marker of fertility. Contraception. (2002): 836-47. [Back]