Can Pregnancy Occur Before Day 15 of LMP?
Yes, pregnancy is absolutely possible before day 15 of the last menstrual period (LMP), because ovulation—and therefore the fertile window—can occur as early as day 9 of the cycle, with conception possible from approximately 5 days before ovulation through the day of ovulation itself.
Understanding the Fertile Window
The critical concept here is that the fertile window precedes ovulation, not follows it. Here's the biological reality:
Ovulation timing varies widely: During an average 28-day menstrual cycle, ovulation generally occurs during days 9-20 1. This means ovulation can happen well before day 15 in many women.
The fertile window is 6 days long: Conception occurs only when intercourse takes place during a six-day period that ends on the estimated day of ovulation 2. The probability of conception ranges from 0.10 when intercourse occurs five days before ovulation to 0.33 when it occurs on the day of ovulation itself 2.
Sperm survival enables early conception: Sperm have been estimated to survive an average of 1.4 days, with a 5% probability of surviving more than 4.4 days and a 1% probability of surviving more than 6.8 days 3. This means intercourse occurring as early as day 9-10 could result in pregnancy if ovulation occurs around day 14-15.
Clinical Implications
For Women Trying to Conceive
Day 14 assumptions are often wrong: Only 12.7% of women correctly estimated their ovulation day in one study, with the median difference being +2 days (range -10 to +27 days) 4. The most common days for estimation of ovulation were day 14 (35.5%) and day 15 (15.7%), but only 55% of estimated ovulation days fell within the volunteers' actual fertile window 4.
Early cycle intercourse matters: Women with shorter cycles or early ovulation can absolutely conceive from intercourse occurring before day 15, potentially as early as day 9-10 of their cycle 1, 2.
For Contraceptive Counseling
The hormone-free interval is high-risk: When two or more consecutive hormonal pills are missed (≥48 hours), emergency contraception should be considered if hormonal pills were missed during the first week and unprotected sexual intercourse occurred in the previous 5 days 5, 6. This recommendation exists precisely because ovulation can occur early in the cycle when hormonal suppression is interrupted.
Extended hormone-free intervals increase pregnancy risk: Studies comparing 7-day hormone-free intervals with shorter intervals found lower pregnancy rates and better ovulation suppression with shorter hormone-free intervals 6. This is because follicular development can begin during the hormone-free week, making early cycle days (days 8-14) particularly risky for conception.
Common Pitfall to Avoid
Never assume all women ovulate on day 14: The "day 21" progesterone testing is based on the assumption of a 28-day cycle with ovulation occurring around day 14 1, but for women with irregular cycles, testing should be performed approximately 7 days before expected menses 1. The assumption that ovulation always occurs on day 14 leads to significant contraceptive failures and missed conception opportunities.
Bottom Line for Clinical Practice
When counseling patients about fertility or contraception, emphasize that the fertile window can begin as early as day 9 of the menstrual cycle 1, 2. Any unprotected intercourse from day 9 onward carries pregnancy risk, particularly in women with shorter cycles or those who ovulate earlier than the "textbook" day 14. For contraceptive users, missed pills in the first week of the pack are particularly high-risk because they coincide with this early fertile window 5, 6.