The morning-after pill: how long does it prevent ovulation for?

The morning-after pill falls into the category of emergency hormonal contraception. It can be obtained free of charge and without a prescription from a chemist. It is used to prevent fertilisation when unprotected sex has taken place or when there has been a mishap with a usual method of contraception, such as a torn condom or a missed contraceptive pill. It works only before ovulation, i.e. before the ovary releases an egg.

There are two active ingredients: levonorgestrel, a synthetic progestogen, and ulipristal acetate, a selective progesterone receptor modulator. The first must be taken within 72 hours of intercourse. The second remains effective for up to 120 hours, or five days. These substances do not cause a termination of pregnancy: they act before fertilisation and are therefore not equivalent to an abortion (1).

In which situations should it be taken?

This emergency contraception is intended for any woman who has had unprotected sex or whose usual contraceptive method has failed. It may be considered:

• if you’ve missed a pill, especially at the start or end of a pack 

• when a condom has broken or slipped off 

• if a vaginal device (ring, diaphragm) has been inserted incorrectly or removed too early 

• or after unprotected sex.

The aim is to prevent fertilisation before it occurs. However, its effectiveness depends on the stage of the menstrual cycle: the pill is only effective if ovulation has not yet taken place.

The principle behind the morning-after pill is to delay ovulation. Ovulation is the release of an egg by the ovary, usually in the middle of the cycle. The two active ingredients available, levonorgestrel and ulipristal acetate, slow down this process by blocking the action of progesterone, a hormone essential for triggering ovulation (1,2). If taken before ovulation, the medication delays its onset until the sperm – which can survive for up to five days in the female reproductive tract – have died off (1). This delay therefore prevents the egg from meeting the sperm.

The morning-after pill does not permanently block ovulation: its effect is temporary and limited to the current cycle. It works by delaying the release of the egg by a few days, just long enough for any sperm present to lose their fertilising capacity. This delay is sufficient to prevent the egg and sperm from meeting. Once this period has passed, ovulation occurs naturally and the cycle resumes its normal course. The morning-after pill therefore has no documented long-term effect on fertility (1,2).

Effectiveness depends primarily on how soon it is taken. Levonorgestrel should ideally be taken within 12 hours of intercourse, and no later than 72 hours. Ulipristal acetate can be taken up to 120 hours – in other words, 5 days – after intercourse. Taking it quickly increases the chances of success: the effectiveness of levonorgestrel ranges from 58 to 95% depending on the time elapsed (2). Certain factors may limit this effect. A body weight of over 75 kg reduces the effectiveness of levonorgestrel, and this becomes virtually zero for weight exceeding 80 kg. Taking certain other medicines at the same time may also reduce the concentration of the drug in the blood (1). Finally, vomiting occurring within 3 hours or severe diarrhoea within 10 hours may prevent the tablet from being absorbed, in which case another dose should be taken.

The side effects observed are usually mild and short-lived. They generally disappear within two days. The most common are nausea, headaches, abdominal pain, and occasionally light spotting between periods or breast tenderness. Some women also report temporary tiredness or slight mood swings.

Your next period may arrive a little earlier or a little later than usual, without affecting your subsequent cycle (1).

As well as the morning-after pill, there is another method of emergency contraception: the copper intrauterine device (IUD). This small device, inserted into the womb by a healthcare professional, can be fitted up to five days after unprotected sex. It works by releasing copper ions, which render sperm inactive and prevent fertilisation. This is the most effective method, as it works immediately and can then be left in place as a long-term form of contraception. The copper IUD is suitable for both women who have never had children and those already using hormonal contraception (1).

The morning-after pill only protects against pregnancy resulting from intercourse that took place before it was taken. Subsequent sexual intercourse during the same cycle remains fertile: it is therefore advisable to use a condom until your next period. After taking ulipristal acetate, resuming your usual hormonal contraception may be less effective. A barrier method is therefore necessary until your period returns. For breastfeeding women, it is recommended to wait between a few hours and a few days before resuming breastfeeding, depending on the active ingredient used (1). Ulipristal is contraindicated in cases of severe liver failure, uncontrolled asthma or lactose intolerance (1).

The morning-after pill should be regarded as a one-off solution and not as a regular method of contraception. It is only effective if taken before ovulation and does not affect fertility. How quickly it is taken is crucial: the sooner it is taken, the lower the risk of fertilisation. If you are unsure about where you are in your cycle, which product to choose, or whether you are currently taking any other medication, seeking advice from a healthcare professional, such as a pharmacist or doctor, is always the best option. It is recommended that you use a condom until your next period.

Stéphanie is a pharmacist (since 2010) and holds a PhD in molecular biology (since 2012). Passionate about writing, she has produced medical content for nearly 15 years. Her goal is to make information accessible and easy to understand, without ever sacrificing scientific accuracy.