What are the chances of pregnancy from unprotected sex the week before a woman's expected period in a 35-day menstrual cycle?

Chances of Pregnancy the Week Before Period in a 35-Day Cycle

The chances of pregnancy from unprotected sex during the week before an expected period in a 35-day menstrual cycle are very low, as this timing falls outside the fertile window for most women.

Understanding Fertility in a 35-Day Cycle

In a 35-day cycle, the timing of ovulation and the fertile window differs from the standard 28-day cycle:

• The fertile window typically consists of the 6 days ending on the day of ovulation 1
• In longer cycles (like 35 days), ovulation occurs later than the commonly assumed day 14
• For a 35-day cycle, ovulation would typically occur around day 21 (approximately 14 days before the next period)
• The fertile window would therefore be approximately days 16-21 of the cycle

Timing and Probability

When considering a 35-day cycle:

• The week before the expected period (days 28-35) occurs well after ovulation
• By this time, the fertile window has closed approximately 7-14 days earlier
• Pregnancy is only possible when intercourse occurs during the 6-day fertile window ending on the day of ovulation 1
• The probability of conception ranges from 0.10 when intercourse occurs five days before ovulation to 0.33 on the day of ovulation itself 1

Important Considerations for Longer Cycles

For women with longer cycles (>32 days):

• The Standard Days Method (SDM) is less effective for women who consistently have cycles outside the 26-32 day range 2
• The timing of the fertile window varies greatly, even among women with regular cycles 2, 3
• Women with longer cycles should not assume ovulation occurs on day 14, as this can lead to inaccurate assessment of fertility 3

Risk Factors and Cautions

Despite the low probability, there are important caveats to consider:

• Individual cycle length can vary from month to month
• The exact day of ovulation can be unpredictable, even in regular cycles
• Some women may occasionally ovulate later than expected
• Sperm can survive in the female reproductive tract for up to 5 days

Emergency Contraception Options

If unprotected intercourse has already occurred and pregnancy prevention is desired:

• Emergency contraceptive pills should be taken as soon as possible within 5 days of unprotected intercourse 2
• Options include ulipristal acetate (30 mg), levonorgestrel (1.5 mg), or combined estrogen-progestin regimens 2
• A copper IUD can be inserted within 5 days of unprotected intercourse as emergency contraception 2
• Emergency contraception effectiveness decreases with time, but remains significant even after 72 hours 4

Summary

The week before an expected period in a 35-day cycle is generally a low-fertility time because:

1. Ovulation has likely already occurred 1-2 weeks earlier
2. The egg survives only 12-24 hours after ovulation
3. Without a viable egg, pregnancy cannot occur regardless of sperm presence

While the probability is very low, those wishing to avoid pregnancy should consider more reliable contraceptive methods rather than relying on cycle timing alone, especially with longer or irregular cycles.