The probability of getting pregnant can be about five times higher 2 days before ovulation than on the day of ovulation itself. Women are said to be at their most attractive on these days. There are several ways to detect them, ranging from free fertility awareness to high-end devices that track physiological and hormonal changes. Before diving in, let's get a few things straight.
6 fertile days per cycle
You can get pregnant only in the five days before ovulation or on the day of ovulation itself. In a 1995 study, 221 women trying to conceive recorded the days of unprotected intercourse, and researchers pinpointed ovulation from estrogen and progesterone in urine samples.1 All 192 pregnancies (129 live births) across 629 cycles were conceived within that six-day window. Since then, we know the fertile window is 6 days long.
The most fertile day
A 2007 study of 728 couples and 5,390 cycles found that fertility peaks 2 days before ovulation, and the probability of conception is very low on the day of ovulation itself.2 Depending on the age group, the chance of pregnancy is about 3–5 times higher two days before ovulation than on the day of ovulation.
Live births
A 1999 study found that timing matters not just for getting pregnant, but for staying pregnant.3 The egg ages quickly — within about 12 hours of ovulation — so sperm need to be already waiting, not arriving late.
The researchers found that the highest chance of live birth came when intercourse happened 1 or 2 days before ovulation. By then, sperm can reach the fallopian tube in time to meet the freshly released egg.
In contrast, intercourse on the day of ovulation was linked to more pregnancy losses. The egg was already aging by the time sperm reached it. Intercourse 3–5 days before ovulation also carried a higher loss rate, because sperm released that early may not survive until the egg arrives.
So while ovulation day is often thought of as the "target," the biology favors a slightly earlier window: the 1–2 days before ovulation, when sperm can be ready and waiting for the egg.
The signs of peak fertility
As estrogen rises, the salinity of body fluids (electrolytes) rises with it. Pay attention to:
- cervical softness and position
- cervical consistency
- increase in mucus and saliva electrolyte levels
- crystallization of salts in mucus and saliva samples (fern patterns)
According to a Harvard Medical School study, the ovulation saliva test can predict ovulation with 99% accuracy by visually recognizing the crystals.4
These signs can appear 3–4 days before ovulation. Then the pituitary releases a surge of luteinizing hormone (LH) and follicle-stimulating hormone (FSH); ovulation typically follows 1 to 1.5 days later. LH shows up in urine about 6 hours after that surge, so LH tests predict ovulation about a day in advance.
Cycle without ovulation?
Sometimes signs are present but ovulation does not happen. This is called an anovulatory cycle and has no fertile window. Its frequency increases with age. Two ovulations in one cycle are also possible in about 1–2% of cases — but both eggs are not always fertilised, which is why fraternal twins are rare.
So when can I get pregnant?
So the real answer is: pregnancy is possible as soon as the body starts preparing for ovulation, and it peaks about two days before — but the exact day can shift, or be skipped altogether. The first signs appear in body fluids. Which means it is not only that women are said to be at their most attractive on these days; their saliva also grows tiny, beautiful fern-shaped crystals. Babyndex is built to spot them.
References
- Wilcox AJ, Weinberg CR, Baird DD (1995). Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby. New England Journal of Medicine, 333:1517–1521. 221 women, 629 cycles; the fertile window is 6 days long.
- Colombo B, Masarotto G (2000). Daily fecundability: first results from a new data base. Demographic Research, 3:5 — analysis of 782 couples and 7,017 cycles, showing conception probability peaks in the days before ovulation.
- Wilcox AJ, Weinberg CR, Baird DD (1999). Time of implantation of the conceptus and loss of pregnancy. New England Journal of Medicine, 340:1796–1799 — timing of intercourse, egg aging, and early pregnancy loss.
- Potluri V, Kathiresan PS, Kandula H, Thirumalaraju P, Kanakasabapathy MK, Pavan SKS, Yarravarapu D, Soundararajan A, Baskar K, Gupta R, Gudipati N, Petrozza JC, Shafiee H (2019). An inexpensive smartphone-based device for point-of-care ovulation testing. Lab Chip, 19(1):59–67. — reports 99% accuracy in predicting ovulation from fern-shaped saliva crystals.