When Did I Conceive Positive Pregnancy TestAre you wondering to yourself; “When did I conceive? What precise day did me having sex result in me becoming pregnant?” It is a question that women ask more than you might think.

Luckily, it is possible to calculate a rough estimate to within 5 or 6 days. I will talk you though how to work this out and provide you with some examples. Unfortunately, it is not possible to be more accurate than this retrospectively because of how long sperm can stay alive with you.


Understanding When Having Sex Can Lead to Conception

Did you know that a man’s sperm can remain alive inside you for up to five days and still fertilise an egg? So anytime you had unprotected sex in the five days leading up and the day after you ovulated (when your ovary released an egg) could have got you pregnant. This is called the “Fertile Window” and it is this that we have to calculate as part of answering the question ‘When did I conceive?’


So, When Did I Conceive?

Can you remember which day the last period you had started? It may require you to sit down and work it out for a while before it comes back to you. Once you know this date, make a note of it as we’ll need it to work out when your fertile window was likely to have happened.

The average menstrual cycle is 28 days long and ovulation occurs roughly two weeks before your new cycle begins, which is around day 14. If your cycle conforms to an average 28 days, add 9 days to the date of your last period and that is the start of the fertile window during which you became pregnant. This fertile window would have lasted for roughly 5-6 days. If you have an irregular cycle, I will cover that in a bit.

Example 28 Day Cycle Calculation

If the first day of your last period started on the 12th of August. This would mean your likely day of ovulation to be around the 26th of August. So, by adding 9 days to the 12th of August would give you a fertile date from the 18th of August through to the 27th of August. Any time you had sex during those 6 days could have resulted in you becoming pregnant.


Calculation for Longer or Shorter Cycles

Unfortunately, not every woman ovulates on a particular day in the cycle, and your ovulation timing can even vary from month to month. This can complicate working out when you did conceive. However, as a general rule of thumb, you still ovulate roughly two weeks before your next period starts. So, if you have a 35 day cycle, you are likely to ovulate around the 21st day of that cycle. Or if your cycle is only 23 days, you are likely to ovulate around the 9th day of that cycle.

Example 25 Day Cycle Calculation

If you started your last period on the 12th of August and had a 25 day cycle, it would mean that your likely day of ovulation was 23rd of August. This would give you a fertile window of 18th of August through to the 24th of August.

You can even work backwards from the day you expected your new period to start, but didn’t.

Example 33 Day Cycle Calculation Worked Back from Expected Date of Period

If you started your last period on the 12th of August and because you have an expected cycle of 33 days, the period you missed should have started on the 14th of September. Taking 14 days off 14th of September would give you an ovulation date of roughly the 1st September. This gives you a fertile window of between 27th August and 2nd of September.


Narrowing It Down to Which Day You Had Sex

Now you have your rough fertility window, to answer the question of when did I conceive, you need to note down all the times you had sex (and possibly with whom) during that time. Any of these encounters could have conceivably resulted in you becoming pregnant.

If you have had multiple partners, it might be worth you knowing that you are more likely to conceive with someone who isn’t your regular partner. This is because in evolutionary terms, we are designed to attract and procreate with men who are not necessarily long-term partners. Though don’t take it for granted that it was your new partner.

In fact, it is even possible to have twins by two different fathers, if you have sex with two different men during your fertile window.


How Accurate Is This Calculation?

Because every woman’s cycle can be slightly different each month and ovulation can happen sooner or later than expected, there can be some difficulties in predicting exactly when having sex got you pregnant. So, it is possible that you became pregnant from sex slightly before or after the predicted fertile window.

If you are unsure, it is best to first discuss this with your doctor. As a doctor myself, I can tell you that it is not as an unusual question as you might expect us to hear, so don’t feel embarrassed by asking it.

Ultimately though, you may have to resort to a paternity test in order to find out exactly who the father of your child is.


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