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When Did You Conceive?

Katlyn Joy | 6, March 2013


The pregnancy test was positive so the big question has been answered. However, the next question may be when did I conceive the baby. For some women this may be a simple calculation but for others it may be a more difficult mystery to solve. Here are some ways to determine when you conceived.

Starting at the Beginning

When dating pregnancies you will often encounter the abbreviation LMP. This means the last menstrual period and refers to the first day of your cycle or the day your period actually began. If you keep good track of such things such as the standard date circled in red on your calendar, you will be ahead of the game in determining the date of conception which in turn helps you be more confident of your due date. If you are a bit fuzzy on the exact date of your LMP, how about the cycle previous to it? You may need this info to help your doctor date your pregnancy.

When Conception Normally Occurs

For most women, ovulation happens around 11 to 21 days after the first day of the cycle or day one of the menstrual period. While many women ovulate between day 14 and day 16, every woman has her own unique biological clock and timing. Making things even more challenging, just because you ovulated on day 15 last month doesn't mean you will again this month. The reproductive processes are individual and fluctuating.

Try the ovulation calculator

You may have some insight into when you ovulate most months by being observant. You may notice abdominal pain around the time the egg is released each month. If you were trying to conceive, you may have been taking your basal temperature and recording it as well as watching the changes in your cervical mucus each month to determine a time of ovulation. Knowing when you ovulate will increase your odds of timing sex just right to get pregnant.

If you've kept a watch and have a good idea of when ovulation occurred and noted this on a chart along with when you had sexual intercourse you'll have an excellent chance of narrowing down your date of conception.

Other Methods for Determining the Date of Conception

If you are unsure on when your LMP was, your doctor may be able to make a pretty good guess based on an early ultrasound. Ideally, you'll want to have the ultrasound performed before the 12th week of pregnancy. The doctor will look at measurements to determine the development of the embryo and give a good estimate of the baby's gestational age.

Another medical option is for your physician to perform a blood test to check the levels of HCG hormone to date the pregnancy. This is only helpful in the early weeks of pregnancy however.

Complications in Determining the Date of Conception

Usually the difficulties in dating a pregnancy occur when a woman is unsure of her cycles or has irregular periods. Should this happen, a variety of methods may be used to make an educated guess such as looking at an early sonogram and going with when the woman thinks her period started and when she had sex the last month.

Another difficulty may be that a woman may have had multiple sexual partners within her fertile window. If sex occurred with more than one partner in a few days time particularly between days 11 and 21, it may mean that a paternity test will be required in order to determine the father.

Simple Way to Determine the Date of Conception

Of course if you have a pretty good idea of your LMP, and an ultrasound that confirms those dates, you will be given a due date.

Barring such special circumstances such as in vitro fertilization where a woman knows exactly when conception occurred, any conception date or due date will be at best a really good guess. Furthermore, babies aren't born on exactly nine months from conception. Babies are individuals with their own sense of timing and you never want to rush baby to be born before the child is fully ready.

Showing 1 - 6 out of 6 Comments
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Sally Oct 11, 2018 04:00:10 AM ET

Hello, i had sex with guy a on march 4-6, than guy b on march 11. my due date is the end of november to december 3. can you help to determine?

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Shinaz Jul 27, 2018 02:01:33 AM ET

I am 24 years old. i am trying to conceive this month. i had sex with my husband on 14th day of this month. my period cycle is 27 days. july 4, 2018 is my last period date. any chances of conceiving this month. please reply. and i am having lower pack pain from 2 days. is it symptom of pregnancy?

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Guest May 25, 2018 03:24:10 AM ET

Hello, i had sex with another guy november 15 and had sex with my boyfriend november 19 and my due date is august 13. who is the dad?

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Lady318 Nov 9, 2017 03:15:14 AM ET

I had sex with guy a on january 5, 2017. got my period on 6 of january through the 9th. got with guy b, had sex on january 13th through the 21st. my due date was october 13, 2017. who's the father?

Steph Mar 16, 2018 01:01:53 AM ET

Guy b

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Mark Mar 24, 2017 10:10:21 AM ET

My girlfriend is pregnant and has an ex partner and they have 3 kids. my girlfriend found out she was pregnant on january 4th 2017. during date was given for 30th august. i have used conception calculators and am convinced the child is mine on those dates as we were official from 15th november. she still has doubts and i am trying to understand what the chances are of the ex being the dad, if she had sex with him previous to 15th november? conception was calculated in 1st week of december. she says she didn't cheat on me. so taking her word, are the odds in my favour?? please help

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Sophie Jun 2, 2013 10:59:21 PM ET

Hi i had sex with someone on october the 11 but then had some bad heavy bleeding for 2 weeks not sure when it was in october thou maybe the 22nd then i had a period which started on 16th or 17th november and then in december i found out i was pregnant my due date is in august who's the dad ????

Guest Jan 10, 2015 01:26:04 PM ET

You should have that answer. we cant tell you.

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