Knowing when you're likely to release an egg (ovulate) can help you plan or avoid pregnancy. Find out when ovulation occurs in the menstrual cycle and when you can get pregnant.
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When can I get pregnant?
During the menstrual cycle, an egg is released from one of your ovaries and travels down the fallopian tube.
The egg only lives for 24 hours after ovulation, and a sperm must meet the egg within that period for pregnancy to happen.
This doesn't mean that a woman has to have sex on the day of ovulation, as sperm can survive in your body for several days after sex.
If you want to get pregnant, having sex every couple of days will mean there are always sperm waiting to meet the egg when it's released.
When does ovulation happen?
It's difficult to pinpoint exactly when ovulation happens, but usually it takes place 10-16 days before the start of your next period.
The first day of your period is day one of your menstrual cycle. The average cycle takes 28 days, but shorter or longer cycles are normal.
"It's not accurate to say that women are fertile on day 14 of the menstrual cycle," says Toni Belfield, a specialist in sexual health information and a trained fertility awareness teacher.
"If your cycle is 35 days long, you're not going to be fertile on day 14 of your cycle. You're more likely to be fertile around days 19-25. If you have a shorter cycle - for example, 23 days - you might ovulate around days 7-13."
Read more about the menstrual cycle and fertility.
It's difficult to know exactly when ovulation happens. So if you're trying to avoid pregnancy, there isn't a "safe" time of the month to have unprotected sex.
"A woman who doesn't want to become pregnant shouldn't take the risk," says Belfield.
For a woman with a shorter menstrual cycle (for example, 23 days) having unprotected sex during her period could put her at risk of pregnancy.
"Sperm can hang around for seven days, and she might ovulate very soon after her period has finished."
Some women use natural family planning to plan or avoid pregnancy. "Working out your fertile time is very dependent on knowing your cycle and knowing your fertility indicators, which takes observation," says Belfield.
The most effective methods of contraception are long-acting reversible contraceptive methods, such as the contraceptive injection, contraceptive implant, intrauterine system (Mirena) and intrauterine device (IUD).
If you've had unprotected sex or your contraception has failed, emergency contraception can help prevent an unplanned pregnancy.
There are two types: the emergency contraceptive pill and the IUD.
The emergency contraceptive pill
There are two kinds of emergency contraceptive pill, also known as the "morning after pill".
Levonelle has to be taken within 72 hours (three days) of sex, and ellaOne has to be taken within 120 hours (five days) of sex.
But it's important to remember that the sooner you take emergency contraception after sex, the more effective it will be. Both work by preventing or delaying ovulation.
The IUD is a small, T-shaped contraceptive device made from plastic and copper. It's inserted into the uterus by a trained health professional.
It may prevent an egg implanting in your womb or being fertilised. The IUD can be inserted up to five days after unprotected sex, or up to five days after the earliest time you could have ovulated.
If you have any questions, you can speak to a pharmacist or GP, or visit a sexual health or family planning clinic.
Find out more about emergency contraception, including where to get it from.
Article provided by NHS Choices