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Women who do not want to conceive a baby but have recently had unprotected sex — or a birth control failure — often find themselves mentally assessing the odds that they will indeed end up pregnant.

This is, of course, closely related to the menstrual cycle, causing SteadyHealth readers to ask questions such as: 

  • Can I get pregnant if I had sex a week before my period?
  • Can I get pregnant if I had sex during my period?
  • Can I get pregnant if I had sex during the first few days after my period ended?

While these are all good questions, they do not address specific women's menstrual cycles and are therefore difficult to offer a general answer to. 

If you have ever had sex ed in school, talked to female friends or relatives about the menstrual cycle, or if you are a "netizen", you'll have a fairly good basic idea of when women can generally get pregnant — during ovulation and the few days before it, because sperm cells can survive inside the female body for about five days. You'll also know that most women ovulate roughly during the middle of their menstrual cycle, and that the fertile window is generally said to occur during that time. 

You'll then answer "probably not" to all three questions above, while still being worried that pregnancy is somehow possible under these circumstances in some cases. Again, the questions you are asking yourself make sense. 

Women's Menstrual Cycles Vary Rather A Bit

Research demonstrates that 95 percent of women have menstrual cycles that last between 15 and 45 days. While you are, as such, incredibly likely to fall somewhere in that range, you can see that the length of the menstrual cycle varies rather a bit from woman to woman. The same study also demonstrates that your menstrual cycle is most likely to be irregular if you are under 25, and that women above the age of 40 once again experience more irregular cycles as they approach the menopause. 

The average cycle length — 28.1 days — is just that, an average. That cycle length is a statistical figure made up of countless menstrual cycles that real women actually experience. The length and regularity of your menstrual cycle both impact your chances of becoming pregnant after unprotected sex at any given time of the month. 

When Does That So-Called 'Fertile Window' Actually Occur?

One study indicated that women have a 10 percent chance of falling within their so-called fertile window on every day between cycle days (counted from the first day of menstruation) six and 21. Between four and six percent of women had occasional "late ovulations", and were potentially fertile during week five of their cycles. 

Only 30 percent of the study participants — definitely a minority — were found to have fertile windows that neatly fit into clinical guidelines.

That means less than a third of all women are fertile only between cycle days 10 and 17, as you will usually read about. Some are fertile earlier in their cycles, with others ovulating later. 

What Does This Mean?

While many women with a luteal phase shorter than seven days (also called a luteal phase defect) find it hard to get pregnant, it does happen.

Essentially, this means that unless you are sure you know when you ovulated and that it was before you had unprotected sex, you indeed have reason to be concerned about unwanted pregnancy after unprotected sex at just about any point of the menstrual cycle. 

Get the morning-after pill within 72 hours if you can, regardless of your menstrual cycle. (As always, if you're worried about sexually transmitted diseases, please get tested for those as well.)

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