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Getting Pregnant

Ovulation Calculators May Cut Pregnancy Chances in Half

Many women use online ovulation calculators as a convenient way cto figure out when to have sex to get pregnant. These calculators estimate your ovulation date based on the rule that ovulation occurs 14 days before your period starts, combined with your average cycle length. So if your cycle is usually 30 days, a calculator will probably tell you that you ovulate on day 16 of your cycle.

These calculators provide a convenient estimate of your fertile days without requiring you to track your fertility signs. But there’s only one problem: for up to 70 percent of women, clinical guidelines about when ovulation occurs are not accurate.

Why is it important to accurately calculate ovulation?

When you want to get pregnant fast, one of the most important things you can do to increase your chances is to have sex during your fertile window, which includes the five days before ovulation and the day of ovulation itself. These are the only six days of your cycle when it’s possible to get pregnant.

But not all six days of the fertile window carry equal chances for pregnancy.

According to one of the largest studies conducted on day-specific probabilities of conception, the three days before ovulation carry the best chances for pregnancy. Having sex on any of these three days carries a roughly 25% chance of conception, but having sex on the other days of the fertile window carries a much lower chance of conception: less than 10 percent. Some smaller studies find that ovulation day carries a high chance of conceiving. Having sex on any of the three days before ovulation means you have more than twice as good a chance of conceiving compared to having sex on the other three days of the fertile window.

What’s wrong with ovulation calculators?

Now you understand why it’s so important to calculate your fertile window accurately and precisely. And when it comes to accuracy and precision, these online calculators just don’t cut it.

Your cycle is a bit like the weather: a forecast can give you an idea of roughly what to expect, but if the weatherman predicts clear skies all week, it won’t stop you from taking an umbrella when see rain out the window. Similarly, these tools can give you a ballpark estimate of when you might be fertile, but to really know what’s happening in your body, there are far more precise and accurate methods of tracking your cycle.

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Can I use an ovulation calculator if my cycle is regular?

Since these calculations rely on a consistent average cycle length to make predictions, they don’t work as well for women with irregular cycles. But what about for women with regular cycles? It depends on what you mean by regular.

If your cycle length varies by just a few days each month, that means that your ovulation date and fertile window also probably varies by a few days per month. Remember that you are trying to identify a peak fertile window of only three days, so being off by a few days could cause you to miss your peak fertile window entirely.

And even if you think your cycle is fairly regular, the fertile window may not fall during the time that clinical guidelines—which ovulation calculators are based on—predict.

What should you use instead?

A much more accurate way to determine when you are fertile is to track your body’s signs and symptoms of ovulation. It helps to understand some basic facts about changing hormone levels during your cycle. There are three hormones you need to know about:

  1. Estrogen: About five days before ovulation, estrogen levels increase. You can tell when estrogen is increasing by tracking your vaginal discharge (aka cervical mucus) (the higher your estrogen levels, the more slippery and stretchy your cervical mucus becomes), or by using a digital ovulation predictor kit (make sure it tracks estrogen as well as luteinizing hormone), or by wearing an Ava bracelet. In the days before ovulation, estrogen levels increase.
  2. Luteinizing Hormone (LH): About 24 hours before ovulation, there is an LH surge that you can detect using ovulation predictor kits (OPKs).
  3. Progesterone: After ovulation, levels of the hormone progesterone increase, causing your basal body temperature to rise. By the time your temperature rises, it’s too late to get pregnant that cycle, but it’s still reassuring to confirm that you actually ovulated.

View sources

Timing of the fertile window in the menstrual cycle: day-specific estimates from a prospective study

Daily fecundability: first results from a new data base.

Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby

Lindsay Meisel

Lindsay Meisel is the Head of Content at Ava. She has over a decade of experience writing about science, technology, and health, with a focus on women's health and the menstrual cycle. Her work has been featured on The Fertility Hour, The Birth Hour, The Breakthrough Journal, and The Rumpus.

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