You Probably Won't Ovulate On The Same Day Each Month, & Here's Why

Personally, I feel like the human body is missing a few features, and one of them is an ovulation alarm. Wouldn't it be great if instead of temping, charting, or buying ovulation predictor kits (OPKs), a little beep went off in your head? Or your big toe glowed? Or something? Unfortunately, the inner workings of human fertility are often invisible — not to mention complex. Questions abound, and here's an interesting one: considering ovulation follows a cyclical pattern, are you supposed to ovulate on the same day each month?

"Individual variation in cycle length, plus the varying lengths of calendar months, means that ovulation is not guaranteed to occur on the exact same numeric date every month," Chase White, MD, an OB-GYN at Einstein Medical Center Philadelphia, tells Romper. Even if your cycle is totally regular, the calendar isn't. Thus, why my birthday doesn't fall on a Friday every year, as I'd definitely prefer. According to White, most menstrual cycles — counting from day one of your first period to day one of the next — last between 25 and 35 days. The second half of your cycle, after you've ovulated, is known as the luteal phase, and it's very consistent. In fact, for a majority of women, the luteal phase lasts 14 days. The wild card here is the follicular phase, when the egg develops. That timeframe is a lot less consistent between women, according to White.

However, the fact that almost everyone shares a 14-day luteal phase is a real boon when it comes to figuring out when you ovulate each month — especially if your periods as a whole are fairly regular. To help you identify which day you'll ovulate, consider an example (details by White, names provided by me):

Anne and Mary Boleyn both start their period on the same day — Jan. 1. Anne Boleyn has 28-day cycles, while Mary has 32-day cycles. Both have very regular periods, and as a result, they can each predict when their next period will make a visit. Anne Boleyn predicts her period will arrive on Jan. 29, while Mary expects hers to come around again on Feb. 2.

While the overall length of their cycles differ, the post-ovulation phase lasts 14 days for both sisters. So Anne can count backwards 14 days from Jan. 29, to predict she'll ovulate on Jan. 15. Mary can count backwards 14 days from Feb. 2, and determine that she'll likely ovulate on Jan. 19.

Likewise, Anne expects her next period on Feb. 26, and ovulation on Feb. 12. Mary will expect her next period on March 6, and to ovulate on Feb. 20. Note that neither sister ovulates on the same date each month, despite the regularity of their cycles, because of the structure of the calendar (Anne expects to ovulate on the 15th in January, but on the 12th come February). If they so choose, the Boleyn sisters can keep playing this game, predicting periods and ovulation dates for months in advance.

But what if you're not as consistent as the two English courtiers in the example above? According to White, "'Normal' menstrual cycles should be fairly regular, with variation in length of less than about a week." Nevertheless, this is a human body we're talking about, so some variance is expected, and certain external factors — like stress, weight loss or gain, and medication — can shake up your cycle every now and again. Usually, you'll return to regular menstrual reality within one or two cycles. If you don't, White suggests visiting your OB-GYN to rule out an underlying health problem.

While you're definitely not supposed to ovulate on the same day each month — in part because calendar months don't all contain an identical number of days — you should ovulate pretty consistently, according to your body's internal rhythm. Discovering that rhythm can be tricky, but it's also well worth it. Using a period and ovulation tracking apps like Natural Cycles (as surely the Boleyn sisters did) for just a few months can help you understand your body better. If what you learn changes your life, or makes you feel more empowered, you can tweet about it on #yourcyclematters. Female fertility may be complex, but it's worth understanding, discussing, and sharing.

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