Females are most fertile within a day or two of ovulation, which is when the ovaries release an egg. But, it is possible to get pregnant in the days leading up to ovulation, as sperm can survive for several days inside the female body.
In this article, we describe how to calculate the fertile window to aid or avoid conception.
The menstrual cycle
The period of fertility depends on when ovulation occurs.
The average person’s menstrual cycle is between 28 and 32 days. Some people have shorter cycles, while others have much longer ones.
The first day of a person’s period is considered the first day of their menstrual cycle. Their period then typically lasts from 3 to 7 days.
Variations in the menstrual cycle usually happen in the follicular phase that occurs before ovulation.
The luteal phase, which occurs from ovulation to the next period, is typically 14 days long.
Ovulation and conception
Ovulation is the release of an egg from one of the ovaries. After release, the egg moves to the fallopian tube, where it stays for 24 hours or so.
Pregnancy occurs if sperm travels to the fallopian tube and fertilizes the egg during this time. If sperm does not fertilize the egg, the egg moves to the uterus and breaks down, ready to leave the body during the next menstrual period.
Calculating ovulation
According to the American College of Obstetricians and Gynecologists, ovulation takes place around 14 days before a person expects to have their next period if their monthly cycle is 28 days.
Most people ovulate between days 11 and 21 of their cycle. The first day of their last menstrual period (LMP) is day 1 of the cycle. Ovulation does not always occur on the same day every month and can vary by a day or more either side of the expected date.
Doctors call the part of the cycle around ovulation the fertile window because a woman’s chance of pregnancy is highest at this time. For example, if a person ovulates on day 14, they can conceive on that day or within the following 24 hours.
However, their fertile window began a few days before ovulation because sperm can survive for up to 5 days inside the female body. So, even if a woman does not have sex on day 14 or 15, it is still possible to become pregnant if they had unprotected sex on days 9 to 13.
A study published in the journal Human Reproduction looked at data from 5,830 pregnant women.
The researchers found that the probability of a person getting pregnant rises sharply 7 days after the LMP. This probability of pregnancy is highest at 15 days and returns to zero by 25 days.
For the women in the study, the probability of being within the fertile window was:
- 2 percent at day 4 of their cycle
- 58 percent at day 12 of their cycle
- 5 percent at day 21 of their cycle
The research also reports that older women and women with regular cycles tend to conceive earlier in their cycle.
It is essential to note that these findings should only act as a guideline. Every person and every cycle is different.
It can be helpful for a person to chart their monthly cycle and take note of the signs of ovulation to help pinpoint the exact day of ovulation each month.
Signs of ovulation
A person’s basal body temperature may increase slightly during ovulation.
Tracking the signs of ovulation can help someone determine the precise day they ovulate each month.
Signs include:
- mild cramping in the lower abdomen
- wetter, clearer, and more slippery vaginal discharge similar to egg white
- a small increase in basal body temperature
- a higher sex drive
Some of these signs, such as basal body temperature, will continue to change after ovulation has occurred. For this reason, a person should not use temperature to predict the fertile window.
It may be helpful for someone to track the signs over a few months to get an idea of what is normal for their body.
But they should keep in mind that there are several variables, and the timing of ovulation can change, month-to-month.
Another option is to use an ovulation predictor kit or fertility monitor.
Fertility aids measure the levels of specific hormones in the urine to determine the ovulation day each month. Some devices also identify days of peak fertility.
Using a combination of these methods may provide an individual with the best accuracy.
The following table summarizes a typical menstrual cycle and how fertile a person is likely to be at each stage:
| Day of cycle | Stage | Fertility |
| 1–7 | Menstruation | Least fertile stage |
| 8–9 | Post-menstruation | Possible to conceive |
| 10–14 | Days around ovulation | Most fertile |
| 15–16 | Post-ovulation | Possible to conceive |
| 17–28 | Thickening of uterine lining | Less fertile — unlikely to conceive |
Maximizing fertility and chance of conception
To maximize the chances of becoming pregnant, a person should time sexual intercourse to occur during the 2 to 3 days leading up to, and including, ovulation. Sex during any of these days may provide a 20–30 percent chance of pregnancy.
Other tips to improve the chances of conception include:
- Have regular sexual intercourse. Pregnancy rates are highest among partners who have sex every 2 or 3 days throughout the month.
- Avoid smoking. Tobacco use reduces fertility and impacts the health of a developing fetus.
- Limit alcohol intake. Alcohol intake can reduce fertility in both males and females as well as harm a fetus.
- Maintain a normal weight. People who are overweight or underweight are more likely to have irregular ovulation.
A doctor can assess a couple’s overall health and may be able to identify methods to improve the probability of conception.
Avoiding pregnancy
Anyone considering the fertility awareness method should speak to a doctor.
Some women may wish to track their fertility to prevent pregnancy. This is known as the fertility awareness method.
The Centers for Disease Control and Prevention (CDC) charts fertility awareness-based methods of contraception as having a 24 percent failure rate with typical use.
The chances of pregnancy are lowest during a person’s period and in the days before and after the period.
However, they may still become pregnant if they have ovulated early or late in their cycle, as sperm can survive in the body for several days.
People wishing to use the fertility awareness method should speak to their doctor first.
Fertility and age
Ovulation and the fertile window can change from cycle-to-cycle, but they may also alter with age. Fertility naturally begins to decline in females from 35 years of age onward.
The number of eggs and egg quality decrease with age. Ovulation may also become irregular.
Some medical conditions, such as endometriosis or polycystic ovary syndrome, also make conception more difficult.
How does fertility differ on the pill?
Birth control pills aim to prevent unintended pregnancy. The pill prevents pregnancy by releasing synthetic hormones that stop ovulation from occurring and the uterus lining from thickening.
So, even if the ovaries do release an egg, a fertilized egg would be unable to implant in the wall of the uterus. The pill also thickens cervical mucus, making it harder for sperm to reach an egg.
According to the CDC, the pill is more than 99 percent effective with perfect use but only 91 percent effective with typical use. This means that, with typical use, around 9 out of 100 women would become pregnant in a year of taking the pill.
When to see a doctor
Women who track their fertile window with the aim of getting pregnant should see their doctor for preconception planning.
As well as identifying barriers to conceiving, a doctor can advise on the use of folic acid or prenatal supplements to encourage a healthy pregnancy.
Most couples who engage in frequent, unprotected sex will conceive within 12 months.
Women under 35 years of age should see their doctor if they do not conceive after a year of trying. Those over 35 years of age should seek medical advice after 6 months of trying to conceive.
Anyone who has irregular cycles or does not appear to ovulate should also speak to their doctor. There may be an underlying medical cause that is preventing ovulation and conception.
Ovulation predictor kits and fertility monitors are available for purchase online.