When you’re trying to get pregnant, what you really need is a fertility calendar: something that tells you the days when having sex is most likely to result in pregnancy.
Of course, you could just have sex and hope it results in a baby, but unless you’re having sex daily or every other day the entire month, that’s a bit like throwing darts with a blindfold on. The “bullseye” when it comes to getting pregnant fast is your six-day fertile window: the five days before ovulation, and the day of ovulation itself. This is the time in your cycle when the possibility of getting pregnant is highest.
Six days may seem like a lot, but not all six of those days carry the same chances for conception. Depending on which of those six days you’re having sex, the odds may be as high as 25 – 30 percent, or just barely higher than zero percent.
Your personalized fertility calendar
So, which days are best? Here they are, ranked in order (“O-2” means two days before ovulation):
Best chance of getting pregnant
O-1, O-2, O-3 (20 – 30 percent chance)
Medium chance of getting pregnant
O-4, O (10 – 12 percent chance)
Low chance of getting pregnant
O-6, O-5 (0 – 5 percent chance)
This data comes from three major studies, each with slightly different findings:
Wilcox NEJM, 19951
The best days from this study are the day of ovulation (32.5 percent chance of conception) and the day before ovulation (30 percent chance of pregnancy).
Wilcox, Human Reproduction 19982
This study, by the same authors as the previous one, found that pregnancy rate is higher with ovulation-day sex, but the chemical pregnancy rate is also higher*. The best days from this study are two days before ovulation (30 percent chance of conception) and the day before ovulation (27.5 percent chance of conception)
Colombo, Demographic Research3
A larger multicenter European study that found the best days are three days before ovulation (27 percent chance of conception) and two days before ovulation (24 percent chance of conception).
How to use your fertility calendar
It’s great to know your odds of conceiving for each individual day within your fertile window. But the major limitation, of course, is that you can’t know for sure when ovulation will occur in advance. It’s much easier to confirm when ovulation occurred in the past than it is to predict precisely when it will occur in the future.
This means that a fertility calendar can be very useful after you’ve confirmed ovulation for assessing how good your timing was.
The studies mentioned above do not all use the same methods for determining when ovulation occurred, which makes it difficult to compare the fertile window between studies. For example, the day considered to be O-1 in the three studies might not actually be the same day, because each of the studies used a different method to identify ovulation (basal body temperature shift, ultrasound, LH test, etc.).
The major takeaway from all this uncertainty? Your best bet is still to simply have sex at least every other day of your fertile window. As soon as you start seeing signs that your fertile window is beginning (like fertile cervical mucus or an fertile window indication on your fertility tracker), you should plan to have sex over the next several days. Don’t wait for a positive ovulation test to have sex, because by the time your test turns positive, you may have already missed some of your most fertile days.
And for any readers who want to really geek out about this stuff, this free journal article provides a detailed perspective on the optimization of natural fertility.
*Why might the chances of chemical pregnancy be higher from sex on the day of ovulation? The authors of the study hypothesize that the difference is because the egg has aged by the time it’s fertilized if sperm are not introduced until ovulation day (because sperm must mature in the cervix for several hours before fertilization, a process called “capacitation”).
- Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby. Allen J. Wilcox, M.D., Ph.D., Clarice R. Weinberg, Ph.D., and Donna D. Baird, Ph.D.N Engl J Med 1995; 333:1517-1521. December 7, 1995. DOI: 10.1056/NEJM199512073332301 ↩
- Post-ovulatory ageing of the human oocyte and embryo failure. https://doi.org/10.1093/humrep/13.2.394 ↩ Human Reproduction, Volume 13, Issue 2, 1 February 1998, Pages 394–397,
- Daily Fecundability: First Results from a New Data Base. Bernardo Colombo, Guido Masarotto. Demographic Research. September 2000. https://www.demographic-research.org/volumes/vol3/5/ ↩