Trying to conceive? Here’s the research on when to have sex.

When you finally decide you want a baby, you pretty much want one now. But babymaking takes time. Sometimes way too much time.

Fortunately, you improve your chances of conceiving quickly with a few simple practices.

Today I want to zero in on the most essential: Timing intercourse correctly.

Know Your Fertile Window

As teenagers, we received dire, and inaccurate, warnings about becoming pregnant. “Sex at any time can lead to pregnancy!

Not so! Conception is only possible during a brief 6-day window each menstrual cycle. To conceive you must have sex at least once—ideally twice—during this 6-day window.

The Evidence for a 6-day Fertile Window

In a prospective study of couples who were trying to conceive, Wilcox and colleagues tracked when during the menstrual cycle a couple had sex relative to ovulation. Over the course of 625 menstrual cycles, 192 couples became pregnant, and, 133 of these pregnancies led to live births.

Using these detailed data, Wilcox found that conception was only possible during a brief 6-day window each cycle. This window opened five days before ovulation and closed the day after ovulation. Chances were highest during the last three days of the window.

(The actual stats behind Wilcox’s analysis are tricky, because couples trying to conceive often have sex multiple times within a women’s fertile window. The researchers therefore had to make some baseline assumptions in their analysis, such as assuming each act of intercourse independently contributes to a couple’s chance of conceiving.)

Graph of probability of conception in relation to day of ovulation
Image taken from Wilcox, et al, 1995

Since Wilcox’s groudbreaking study, several other studies have confirmed the 6-day fertile window.

Age and the Fertile Window

Dunsen and colleagues tracked pregnancies among 782 couples using natural family planning methods, focused on the effect of age on the fertile window. Age had no impact on the duration of this window (It lasted 6 days in women of all ages), but older couples’ chances of pregnancy on each fertile day were lower.

Graph of probably of conceiving relative to the day of ovulation
Image from Dunsen et al., 2002

(Note that this graph differs slightly from Wilcox’s: It shows a peak two days prior to ovulation and it shows a small chance (about 10%) of conception the day after ovulation, probably because Dunsen’s study used basal body temperature to determine the day of ovulation, a less accurate method than the one used by Wilcox (tracking urinary hormones).

Fertility began gradually declining in women after age 26, whereas fertility in men declined noticeably only after age 35. (Women’s and men’s ages tend to correlate, so researchers could only examine the effect of older men when paired with older women.)

Graph showing the chances of conceiving in relation to day of ovulation and age of the woman
Image from Dunsen et al., 2002

But members of the 30-plus set should not view these results as discouraging. Older couples can overcome much of their disadvantage simply by timing intercourse correctly.

In a prospective study of 2820 Danish couples, for example, 87% of 30-to-34-year-old women conceived within a year, compared to only 72% of 35-to-40-year old women. But timing intercourse provided a bigger boost for these “older” women. Seventy-eight percent of 35-to-40-year-old women who timed intercourse conceived within 12 cycles.

How to Time Intercourse Correctly?

Ovulation can be unpredictable, even if your cycle is like clockwork.

Thus, the simplest way to ensure you have sex within your fertile window is to have sex at least three times a week. Frequent sex throughout your cycle prevents later or earlier than expected ovulation from derailing your plans.

But not everyone has the stamina or time to manage this coital feat. Work, stress, travel, and young kids can make frequent intercourse logistically challenging, not to mention undesirable. No one wants their sex lives to become a regimented military exercise.

Fortunately, having sex at least twice within the fertile window is just as effective as frequent sex throughout the month.

If even that seems daunting, the science is clear on which single day of your cycle to target: One to two days prior to ovulation.

By aiming to have sex 1-2 days prior to ovulation, you are virtually guaranteed to hit one of your three peak days, even if you overshoot by a day.

If you instead aim for the day of ovulation and overshoot by a day, you can miss the optimal window entirely.

And there is another advantage: Sperm deposited in a woman’s reproductive tract before ovulation have the best chances of fertilizing a newly released egg.

Some older data implied that eggs were fertilizable for 24 hours after release. We now know this is untrue. After release from the ovary,  the egg begins to “age”—rapidly accruing damage.

In IVF, in which multiple eggs are first retrieved and then exposed to sperm in the lab, fertilization rates are highest within 6 hours of retrieval.

In Wilcox’s study, a similar phenomenon may have occurred. Women who conceived on the day of ovulation, rather than before, were more likely to experience a miscarriage, perhaps because the egg had “aged” prior to being fertilized.

Screen Shot 2019-04-03 at 4.01.19 PM

Can you time intercourse to raise your chances of having a boy or a girl?

Fraternal Twin Baby Brother and Sister

A popular myth (sometimes called the Shettles Method)  states that couples who have sex closer to ovulation are more likely to conceive boys.

The theory behind this claim is that boy sperm are faster but more fragile than girl sperm. If you deposit them at the last minute, they reach the egg first. But they are less likely to survive several days prior to the egg’s release.

It’s a nice story. Unfortunately, the available scientific evidence does not support it.

How to Pinpoint Your Most Fertile Days

To time intercourse correctly you have to know when you’re fertile.

Many methods exist for detecting ovulation—the calendar method, tracking changes in your basal body temperature or cervical mucus, fertility monitors, and ovulation test kits. The sheer range of options can feel overwhelming.

However, three of these approaches clearly work best:

  • Tracking changes in your cervical mucus (free!)
  • Ovulation test kits (pricey)
  • Fertility Monitors (expensive, but nothing compared to the cost of having a baby!)

Ovulation Test Kits

Ovulation test kits detect the surge in luteinizing hormone (LH) that precedes ovulation.

Ovulation test kits work best if you have sex after your first positive reading. Meaning before you get a positive, you need to get a negative. In practice, this means you need to start testing several days before you expect to ovulate.

And this fact feeds into the major downside of ovulation tests: They are fairly pricey, especially for women with irregular cycles.

And they have another problem: The surge of LH can occur anywhere from 16-48 hours prior to ovulation. If you are on the shorter end of that range, and you test only once a day, you could miss the optimum window for timing intercourse.

Tracking Changes in Your Cervical Mucus

Tracking changes in your cervical mucus is one of the best and most reliable indicators of ovulation. As women near ovulation, their mucus changes from dry (not fertile) to damp to creamy to egg white (most fertile).

Peak fertile quality cervical mucus is wet, slippery, and has the texture and appearance of raw egg whites.

Image of peak fertile quality cervical mucus
Fertile quality cervical mucus

(For more info on how to track your cervical mucus, check out Toni Weschler’s amazing Taking Charge of Your Fertility, the bible of the fertility world.)

Having sex during peak cervical mucus boosts your chances of pregnancy more than merely having sex close to the day of ovulation. This is especially true for couples with impaired fertility.

Sex on days with peak mucus also appears to undo some of the detrimental effect of older male age on the chances of conception.

For those who have trouble reading their cervical mucus—and frequent sex can definitely make this more difficult—fertility monitors are another highly accurate (but costly) option. These monitors use urinary hormones to pinpoint your fertile window.

For more science-backed conception tips, check out my earlier post.

 

 

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