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Why is There a Risk of Pregnancy If the Combined Oral Contraceptive Pill is Missed?

It helps the patient to understand the risk of pregnancy if the GP explains why she is at risk if pills are missed and what to do if this occurs. The pregnancy risk changes depending on where in the pack cycle the pill or pills are missed. It is actually very logical if one understands the physiological explanation, but most patients will have trouble recalling this information without a written reminder (author’s original research).

Firstly, a missed pill is now defined as taking the combined oral contraceptive pill more than twenty-four hours late (it used to be twelve hours late). The ultra-low dose combined contraceptive (20ug of oestrogen) used to be defined as being taken late after six hours. A missed pill should be taken as soon as remembered, even if this means taking two active tablets within twenty-four hours.


It takes an absolute minimum of eight days for a human ovum to develop and be ready for ovulation. A somewhat disconcerting fact is that the development of the ovum commences during the time the woman takes the inactive (sugar) pills as there is no continuous active hormones to prevent this. There are seven sugar pills in the older oral contraceptives, and impending ovulation is halted by the recommencement of the active tablets. This explains why contraception is most assured when the contraceptive pill is used continuously (i.e. the sugar pills are not taken because the active tablets are consecutively used over months).

This also explains why the most dangerous time to miss a pill is in the seven days before and after the sugar pills, especially in the older pill types that have seven days of inactive tablets.  The initial trigger for the egg to develop requires a missed pill and if this occurs in the seven days prior to the inactive tables the following scenario could occur. Ovulation could take place at some stage during the inactive tablets. Sperm can live three to five days inside the female and so the window of pregnancy risk can be significant if sex takes place just before or during timing of the inactive tablets. It is a common fallacy that the risk of pregnancy is always at the time of the missed pill. The good news is that this is very preventable by skipping the inactive tablets and continuing for the next month with the active tablets from the next pack.

This also means that the safest time to miss a pill is in the middle of the active tablets section, as more than eight active tablets will follow to prevent ovulation and more than eight have occurred since the sugar pills (when the egg may be developing). However, the most dangerous time to miss an active pill is in the seven days after the inactive tablets. The opportunity for skipping the inactive tablets and continuing on with active tablets has gone, and if the patient has been sexually active in the three  to five days prior to missing the pill, the only thing she can do to prevent a pregnancy is to take the morning after pill or have an IUD inserted within five days.

When the reasons for ovum development and ovulation are understood, the apparently complex guidelines for what to do in the event of a missed pill suddenly become very logical. The following site contains a printable summary about missed pills for patients.

Family Planning NSW