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When Did I Conceive?

by Katlyn Joy | May 12, 2011

While actual dates of conception are difficult to determine because of the number of biological unknowns, it generally is fodder for conversation or guesstimating. However, it's fun to try to find out if it happened that long weekend in the mountains or when you made up after that silly fight you had over cleaning out the garage.

Conception Math

  1. Take the date of your last menstrual period. You count this as the first day of your last period, by the way.
  2. Add 12 days to this date as this is an average range of when ovulation would occur in the typical menstrual cycle.
  3. Add another 10 days to this date and highlight those 10 days. Which of those days were you and your sweetie getting busy? If only one time in that stretch did you have intercourse, you have found your likely conception date.

Those who practice fertility charting and chart their temperatures have been seriously working at getting pregnant probably know when they most likely ovulated during their cycle by looking at the records of basal body temps and observations of their cervical mucus. Women who have predictable cycles have an even better likelihood of knowing when they actually got pregnant. Additionally, those who chart usually also keep track of the dates they had sex.

Factors that Throw Off the Calculations

Women who recently stopped taking birth control pills or used hormonal methods for birth control may not have resumed a regular cycle yet and will have difficulty pinpointing the time of ovulation. Also, those who don't have regular periods or are not the best record-keepers of when cycles occur will be at a definite disadvantage than other expectant mothers when trying to determine when baby was made.

Sperm Facts

Of course, while the egg is the focus of attention here, let's not forget that crucial ingredient to baby-making; sperm! Sperm can throw off calculations by virtue of the simple fact that they don't live just a day in many cases. Indeed, sperm can often survive a day or two, and may live up to five days. So while you may have had sex only one time in the week you conceived you may not have gotten pregnant the day you had sex. It may have occurred a couple days later.

Couples who have more sexual activity together may have a more difficult time determining the day of conception. If they only had sex once that month, well it doesn't take a special formula to determine the special time. However, couples that are more amorous have more possibilities to consider.

Hard Facts

Even with a completed chart of temps, mucus observations, dates of when sex occurred, and the last menstrual period it can be impossible to pinpoint the exact moment of conception.

Early ultrasounds coupled with the firm date of the last period however can nail down a fairly precise due date, and a corresponding likely date of conception. However, the technology is less precise as the pregnancy progresses and a baby who grows at a different than typical pace can throw off results as well.

If you and your partner are beginning the baby-making adventure and you want to know when you achieved your goal, keep careful records of your periods and keep a sex calendar as well. Consider an ovulation predictor test as well to help with more exact date. There are plenty of online due date calculators, which in reverse determine your date of conception, as well. Plus, consider some storyteller liberties when talking of that special night when you two became a family.

Implantation Calculator - Find out when implantation occurred

Ovulation Calculator - Find out when you ovulated

How is Pregnancy Due Date Calculated?

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Showing 1 - 10 out of 38 Comments
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nina Jan 22, 2015 11:34:15 AM ET

I had sex on november 24, 2014. when is my due date? help me figure this out please.

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kim Jan 14, 2015 07:55:47 PM ET

I had my son on nov 11, 2014. can anyone help me find out around what day and month i got pregnant? my last period started feb 8th, and lasted until feb 13, 2014.

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Diana Dec 29, 2014 11:04:28 PM ET

My due date is may 25, 2015. when do you think i conceived?

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guest Nov 14, 2014 06:02:31 PM ET

I'm due on december 4 2014. when did i get pregnant?

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busi Nov 13, 2014 06:13:07 AM ET

Hey. i am 11 weeks and 2 day. when did i conceive?

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nita Oct 16, 2014 12:48:09 AM ET

I'm 8 weeks and 7 days pregnant. when did i conceive, and when is my due date? my doctor hasn't told me that just yet.

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kenya Sep 9, 2014 03:42:15 PM ET

If my due date is feb 11th, when was my baby conceived?

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Guest Sep 8, 2014 02:15:43 PM ET

On friday 09-05-2014 i was 8 weeks pregnant. when did i conceive?

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Missy Jul 27, 2014 10:26:46 AM ET

I had unprotected sex with my son's father on memorial day. i came on my period june 11th. i was told that i am 6 weeks and 6 days pregnant as of today (7/27/2014). how is that possible?

Jasmine Oct 24, 2014 06:14:39 PM ET

The first day of my lmp was on june 11 2014, but the doctors are telling me i am 20 weeks as of 10-24-2014. how is that possible?

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can anyone help please! May 29, 2014 07:19:54 PM ET

Can anyone help? so i did an opk test on the 23rd of may and the smiley face indicated i was at peak for ovulation.( which says it lasts for 2 days) my first day of my period was may 12th. i had intercourse with my hubby on the 19th which i had some spotting. we then had intercourse on the 24th and on the 27th. for giggles i took an opk today and it said i was at peak, so i'm confused. am i pregnant or not?

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