When are you more likely to conceive?
We’re talking about the 'fertile window’ – the days in a woman’s menstrual cycle when pregnancy is possible. The ‘fertile window’ depends on the length of the menstrual cycle, which varies among women.
The ‘fertile window’ is the day an egg is released from the ovary (ovulation) and the five days beforehand. Having sex (intercourse) during this time gives you the best chance of getting pregnant.
A woman can get pregnant any time of the month.
A woman can only get pregnant on a few days during her menstrual cycle.
Because eggs and sperm only live for a short time:
- Sperm live for around five days.
- Eggs can only be fertilised for around 24 hours (one day) after being released from the ovary.
Eggs and sperm need to come together at the right time for fertilisation to happen to create an embryo.
Facts about timing
Ovulation is when a mature egg is released from the ovary. The egg then moves down the fallopian tube where it can be fertilised. If sperm are in the fallopian tube when the egg is released, there is a good chance that the egg will be fertilised, creating an embryo, which can grow into a baby.
Pregnancy is technically only possible if you have sex during the five days before ovulation or on the day of ovulation. But the most fertile days are the three days leading up to and including ovulation. Having sex during this time gives you the best chance of getting pregnant.
By 12-24 hours after ovulation, a woman is no longer able to get pregnant during that menstrual cycle because the egg is no longer in the fallopian tube.
There’s almost no chance of getting pregnant if you have sex before or after the fertile window (but if you’re not trying to get pregnant, don’t rely on this – contraception is your best option!).
How to know when you’re ovulating
Knowing when you ovulate can help you plan for sex at the right time and improve your chance of getting pregnant. You can keep track of your menstrual cycles on a chart, in a diary, or on a free period-tracker app on your smartphone.
To work out the length of your menstrual cycle, record the first day you start bleeding (first day of your period). This is day 1. The last day of your cycle is the day before your next period begins.
What is a ‘menstrual cycle’ and a ‘period’?
Some people think the ‘menstrual cycle’ and a ‘period’ are the same thing.
A period is when you bleed (or menstruate).
A menstrual cycle starts on the day when a period starts (day 1) and ends the day before the next period. A cycle’s length is considered normal if it’s between 21 and 35 days. They can vary between women and from one cycle to the next.
Working out your ‘average’ menstrual cycle length
If your menstrual cycles are different lengths (most women’s cycles are) you can work out your average cycle length.
The number of days in a woman’s menstrual cycle can vary month to month. Periods are not always regular. It can be useful to work out an ‘average’ cycle length, based on the length of three menstrual cycles, to estimate when you’re most likely to be ovulating.
If you add the number of days in three cycles and divide the total number by three, it gives you your average cycle length.
Sarah tracked her last three menstrual cycles by counting the time from the first day of one period, to the day before the next period.
Cycle 1 was 28 days; Cycle 2 was 32 days; Cycle 3 was 27 days
28 + 32 + 27 = 87
87 divided by 3 = 29
So the average length of Sarah’s menstrual cycles is 29 days.
Working out your most fertile days
When you know your average menstrual cycle length, you can work out when you ovulate.
Ovulation happens about 14 days before your period starts.
- If your average menstrual cycle is 28 days, you ovulate around day 14, and your most fertile days are days 12, 13 and 14.
- If your average menstrual cycle is 35 days ovulation happens around day 21 and your most fertile days are days 19,20 and 21.
- If you have shorter cycles, say 21 days, ovulation happens around day 7 and your most fertile days are days 5, 6 and 7.
Your most fertile days are the three days leading up to and including the day of ovulation.
Some women have very irregular cycles or find it difficult to work out an average cycle length. This can make it hard to work out when ovulation happens. If it’s all too hard, having sex every 2-3 days covers all bases and improves your chance of getting pregnant.
Other ways to work out when you ovulate
This ovulation calculator can help you work out your most fertile time. These are the days you are most likely to get pregnant.
It can also estimate your due date if you do become pregnant during your next fertile days.
Others ways to help you work out when you're ovulating:
Notice changes in vaginal mucus
A few days before ovulation, you may notice your vaginal mucus becomes clear, slick and slippery, and feels a bit like egg white.
This is a sign that ovulation is about to happen. It’s the best time to have sex, as sperm travel more easily in this kind of mucus.
Use an ovulation predictor kit
You can use a predictor kit from a supermarket or pharmacy, to test your urine for signs of ovulation. If you start testing your urine a few days before the day you next expect to ovulate, a positive result means you are going to ovulate within the next 24 to 36 hours (one to two days).
What are the chances?
Having sex as close as possible to the time of ovulation increases the chance of pregnancy.
If a woman has sex six or more days before she ovulates, the chance she will get pregnant is virtually zero.
If she has sex five days before she ovulates, her probability of pregnancy is about 10 percent.
If she has sex on the day of ovulation, or the two days before, the chance of getting pregnant is around 30 percent.
- American Society for Reproductive Medicine, Optimizing natural fertility, https://www.reproductivefacts.org/news-and-publications/patient-fact-sheets-and-booklets/documents/fact-sheets-and-info-booklets/optimizing-natural-fertility/
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Page created on: 28/08/2018 | Last updated: 14/11/2018