Conception Probability Calculation
For this 28-year-old woman with confirmed ovulation and a normal HSG who had intercourse at -4, -1, and +1 days relative to ovulation, the per-cycle conception probability is approximately 25-30%, with the intercourse at -1 day before ovulation contributing the most significantly to this estimate.
Breakdown of Day-Specific Conception Probabilities
The timing of intercourse relative to ovulation is the critical determinant here. Based on prospective fertility studies:
- Intercourse at -4 DPO (4 days before ovulation): Contributes minimally, approximately 8-10% probability of conception from this act alone
- Intercourse at -1 DPO (1 day before ovulation): This is within the peak fertile window and contributes the highest probability, approximately 30-35% from this act alone
- Intercourse at +1 DPO (1 day after ovulation): Contributes very little to no probability, as the egg is typically no longer viable for fertilization beyond 12-24 hours post-ovulation
Key Factors Supporting This Estimate
The fertile window spans approximately 6 days ending on the day of ovulation 1. The day before ovulation represents one of the highest probability days for conception in this window.
The normal HSG finding is particularly relevant here. Recent evidence demonstrates that HSG itself increases ongoing pregnancy rates compared to no HSG, with an adjusted hazard ratio of 1.48 (95% CI 1.26-1.73), corresponding to an absolute 6% increase in 6-month pregnancy rate 2. This therapeutic effect occurs regardless of whether water- or oil-based contrast was used, suggesting your patient may have a slightly enhanced baseline fertility in the cycle following the HSG.
Age-Related Fertility Context
At 28 years old, this patient is in an optimal fertility age range. Day-specific probability data from spontaneous pregnancies shows that conception probability sharply rises at 7 days after last menstrual period, reaching maximum at 15 days, with the fertile window having maximum probability of 58% at Day 12 3.
Confirmed Ovulation Significance
The rising progesterone level confirms ovulation occurred. In conception cycles, midluteal progesterone values average 40.7 nmol/L (12.8 ng/ml) with a range of 27-53 nmol/L, and values above 30 nmol/L (9.4 ng/ml) provide a clinically reliable criterion of potential fertility 4.
Clinical Caveats
- The cumulative probability from multiple acts of intercourse is not simply additive—having intercourse on multiple days doesn't multiply the chances proportionally
- The -1 DPO intercourse timing is the primary contributor to conception probability in this scenario
- Individual cycle variation exists, but with confirmed ovulation, the timing assessment is more reliable
- Per-cycle fecundability (monthly conception rate) for healthy couples averages 20-25%, and this patient's timing places her at the upper end of this range