The probability of conception on different days of the cycle with respect to ovulation: an overview

Several mathematical models have been developed over the past thirty years to investigate how the probability of conception changes on the different days of the cycle with respect to ovulation. A problem general to all models is to estimate the day of ovulation. Since the most fertile days are those close to ovulation, less precise estimates of this event will lead to less accurate estimates of the probability of conception on a given day of the cycle. Given that a reference point for ovulation is available, the first model considered conception as dependent only on the timing of intercourse. Conception was found to be most likely to occur on only six days in each cycle. However, the model is biologically unrealistic because it assumes that all ova can be fertilized and lead to a viable pregnancy. There are other factors that affect the probability of conception, including whether the ovum is viable or not. Recent models have extended the idea of cycle viability to allow for differences between cycles within couples and for the introduction of couple specific covariates. In a second group of models the probability of conception depends mainly on the time of intercourse and the survival times of sperm and ovum. A graphical summary of the results available in the literature is presented. Conception probabilities have been found to be significantly different from zero from five days before ovulation to the day of ovulation itself. On average, less than half of the cycles are viable in women, although recent studies suggest that different cycle viability between women should be taken into account. Survival times for sperm and the ovum have been estimated to be 1.4 days and 0.7 days, respectively. Sperm would have a 5% probability of surviving more than 4.4 days and a 1% probability of surviving more than 6.8 days.