When to have sex to get pregnant

Knowing your body and how it changes can help you understand your fertile window.
Knowing your body and how it changes can help you understand your fertile window. Photo: 1234rf

For couples who are trying to conceive, the chance of it happening within a year all depends on their age. When the woman is 35 years or younger, the monthly chance of pregnancy is about 20 percent. Of these, 80-90 per cent become pregnant within 12 months of trying.

By age 40, the monthly chance has dropped to 5 per cent, and half of couples conceive within 12 months.

The age of the father-to-be matters too: women with partners aged 45 or older are almost five times as likely to take more than a year to conceive compared to those with partners in their 20s.

While there is little we can do about our age, timing sex to coincide with your most fertile days can reduce the time it takes to conceive, and could even save you a trip to the fertility clinic.

Finding your fertile window

At ovulation, an egg is released from the ovary. If there is sperm waiting around at that time, there is a good chance it will be fertilised and grow into a baby.

Conception is only possible from about five days before ovulation through to the day of ovulation. These six days are the “fertile window” in a woman’s cycle, and reflect the lifespan of sperm (five days) and the lifespan of the egg (24 hours). But the likelihood of conceiving is dramatically increased if sex occurs in the three days leading up to and including ovulation.

The fertile window varies depending on the length of a woman’s cycle: the time from ovulation to the next period is approximately 14 days, irrespective of cycle length, but the time before ovulation varies between women.

  • In a 28-day cycle, ovulation typically occurs around day 14, and the chance of conceiving is greatest between days 11 and 14.
  • In longer cycles, say 35 days between periods, ovulation happens around day 21, and days 18 to 21 are the most fertile days.
  • In a 24-day cycle, ovulation happens around day 10, and the most fertile days are days seven to 10.

If a woman has sex six or more days before she ovulates, the chance she will get pregnant is virtually zero. Then the probability of pregnancy rises steadily. If she has sex in the three days leading up to and including ovulation, she has a 27-33 per cent chance of becoming pregnant.

From that point, the probability of pregnancy declines rapidly. A woman is no longer able to get pregnant during that cycle from 12 to 24 hours after she ovulates.

Knowing your body and how it changes when ovulation approaches is also important. A few days before ovulation, the vaginal mucus changes and becomes clear and slippery (the consistency of egg white), which is perfect for sperm to swim along. Mucus changes provide an early and useful cue that ovulation is approaching.

If you want to reassure yourself about getting your timing right, you can use ovulation predictor kits which are available at pharmacies and supermarkets. A few days before you think you'll be ovulating you can start testing your urine each day; you can expect to ovulate 24 to 36 hours after the test turns positive.

If all this seems too complicated, an alternative is to have sex every two to three days. That way all bases are covered without getting too technical about when the chance of conceiving is greatest.

Track your fertile windows and ovulation times with the Essential Baby Egg Timer. You can get it for free from the iTunes App Store

This article first appeared on The Conversation.