Can pregnancy occur the week before menstruation?

Pregnancy Possibility in the Week Before Menstruation

Yes, pregnancy can occur in the week before your expected period, though the chances are lower compared to mid-cycle. This is because ovulation timing can be highly variable and unpredictable, even in women with regular cycles.

Understanding the Fertile Window and Ovulation

According to CDC guidelines, the timing of ovulation varies significantly among women and even between cycles in the same woman 1:

• In a typical 28-day cycle, ovulation generally occurs during days 9-20
• The "fertile window" includes the day of ovulation and the 5 days before it
• Most women cannot accurately predict when they will ovulate, even with regular cycles
• Only about 30% of women have their fertile window entirely within days 10-17 of their cycle

Why Pregnancy Can Occur Before Your Period

1. Unpredictable Ovulation: Even in women with regular cycles, ovulation timing can vary significantly 2

   • On any day between days 6-21 of the cycle, women have at least a 10% chance of being in their fertile window
   • Late ovulation can occur unexpectedly, even in women with typically regular cycles

2. Sperm Survival: Sperm can survive in the female reproductive tract for up to 5 days

   • This extends the fertile window beyond just the day of ovulation

Probability of Pregnancy Before Period

The likelihood of getting pregnant in the week before your expected period depends on several factors:

• Cycle Length: Shorter cycles mean ovulation occurs closer to your period
• Cycle Variability: Women with highly variable cycles have a 51% lower probability of pregnancy per cycle 3
• Timing of Intercourse: The highest probability of conception occurs with intercourse 1-2 days before ovulation 2

How to Know if You're Pregnant

If you've had unprotected intercourse in the week before your expected period:

1. Pregnancy Testing: Home pregnancy tests can detect pregnancy as early as the first day of a missed period 4

   • Blood tests can detect pregnancy 7-10 days after conception

2. Early Pregnancy Signs: The CDC notes that a gestational sac is typically visible on ultrasound at 5 weeks gestation, with a yolk sac visible at approximately 5½ weeks 4

Important Considerations

• Menstrual Cycle Tracking: The CDC recommends that women who wish to avoid pregnancy should be advised that their fertile window can be highly unpredictable 1

• Emergency Contraception: If you've had unprotected intercourse within the last 5 days and wish to avoid pregnancy, emergency contraception may be an option 1

• Cycle Characteristics: Sexual intercourse during a cycle is associated with cycle characteristics suggesting higher fertility, including longer luteal phases and more days of cervical fluid with estrogen-stimulated qualities 5

Bottom Line

While the probability decreases as you approach your period, pregnancy can still occur in the week before menstruation due to the unpredictable nature of ovulation. If you're trying to avoid pregnancy, reliable contraception should be used throughout the entire menstrual cycle, not just around the presumed time of ovulation.