The reality: the timing of ovulation is unpredictable

After UPSI sperm can survive for approximately 5 days within the female reproductive tract.1 This means that during the average woman’s menstrual cycle there are six days when intercourse can result in pregnancy; this ‘fertile window’ is the five days before ovulation plus the day of ovulation.2

Six day fertile window2


So when is the fertile window? Current evidence challenges the simplified ‘text book’ understanding of the menstrual cycle.2 We now know that ovulation happens on day 14 of a 28 day cycle2 in about 12% of cases.3

Only about 12% of ovulations happen on day 14.3  The variability of ovulation is large – it can happen from day 11 to day 21.2 Because sperm stay viable for up to 5 days1, the period over which conception is likely to occur runs from day 6 to day 21 for regularly cycling women.2 If the cycle is not regular, there is a risk of ovulation happening even later in the cycle.2 The conception risk period does not end before day 28 of their cycle.2 This shows that there is no such thing as a risk free period.2

Ovulation also varies from cycle to cycle.2

7.Web ovulation is unpredictable

Although the risk of pregnancy exists most of the time,2 women may underestimate the risk of pregnancy.4 This lack of awareness of pregnancy risk may be the most important barrier to emergency contraception use5

The highest risk of pregnancy is when ovulation happens shortly after UPSI6

Sperm viability declines over time. This means that the risk of conception is highest during the first three days following unprotected sex or contraceptive failure.6


Therefore, to avoid unwanted pregnancy, it is critical to avoid ovulation (happening shortly after UPSI) by using EC as soon as possible.


1.Pallone SR and Bergus GR. JABFM 2009; 22(2): 147-157.
2.Wilcox AJ et al. BMJ 2000; 321: 1259-62.
3.Baird DD et al. Epidemiology 1995 ; 6 : 547-550.
4.HRA Pharma Report. Women and emergency contraception in 2012.  A European Survey.
5.Moreau C et al. Contraception 2005; 71: 202-207.
6.Wilcox AJ et al. N Engl J Med 1995; 333: 1517-21.