How accurate are conception dates?

Lily - posted on 08/05/2013 ( 8 moms have responded )




I've had a few ultrasounds all at different places and each one says my conception date is the same as the last. I'm asking this because the day I supposedly conceived my bf and I were in a rush and he didn't even have time to finish. We also used a condom. I didn't have sex the day before my conception date either.


Michelle - posted on 08/05/2013




The way they do the dates is from the start of your last period, not conception. if you are 6 weeks then it could only be 4 weeks since you had sex but you started your last period 6 weeks before.

Jodi - posted on 08/06/2013




They are also not particularly accurate, definitely not to the day. It all comes down to your cycle, when in the month you actually conceived, when in the month you ovulate, and so on. The doctors don't calculate based on the day you had sex.

Emma - posted on 08/08/2013




Sperms live for 3-6 days inside of you waiting for the egg to drop. So If you had sex any time the week befor the ovulation date (wich is 2 weeks after your pregnancy starts) it will still be on the "spot".

[deleted account]

I guess I'm not sure why this matters, unless there is a paternity question. You could have gotten pregnant without your boyfriend "finishing" - there is usually pre-cum that has sperm in it, but maybe it was a few days before or after.

I'm also wondering - in my experience ultrasounds never give you a "conception date", only a due date. Pregnancy is counted as 40 weeks from the start date of your last period. When conception actually occurred depends on when you ovulated. The normal 40 weeks assume that you have a 4 week cycle, which is not true for everyone. The due date given by an ultrasound is based on the size and development of the baby. If your baby is a little big or a little small, it might change the ultrasound's due date by a few days.

Holly - posted on 08/09/2013




Ovulation dates based upon ultrasound machine measurements have a plus or minus, an average machine, of 7 days. Now three different machines getting the same date, but on different dates with different techs still gonna be up to 7 days off. So not that terribly accurate when you think about it. For example in the end of pregnancy when they calculate weight with the ultrasound machine it has a plus or minus of two pounds! So it's not terrible inaccurate but when you want to get specific like let's say one single conception day. Yeah not so good for that. You have a range.


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Melissa - posted on 08/10/2013




Wow it doesnt matter if you are only having sex with one man(; sry if sounds rude. True though .

Martha - posted on 08/09/2013




days before, or after. Sperm can live for DAYS and your egg is good for a couple days. It is a guess. I ovulated 10 days after my first day of period, thus 4 days early for the "guess" date. They go by your period, not true ovulation. It is usually within a week of your "guess" date. Good luck!

Mariya - posted on 08/08/2013




In addition to everything said, the length of your cycle also also affects when the future baby was actually conceived. For example, if your cycle is 32 days, then the conception should be on the 18th day (not on the 14th day, unlike in a 28-day cycle). Some providers do correct for it, but not all.
And as mentioned before, sperm lives for 3-5-7 days.
Ultrasounds are not accurate. The ultrasounds I had (a total of three) estimated the day of deliver with a margin of error of 2-4 days. And, btw, I gave birth later than it was estimated by those ultrasounds (but right on time when it was estimated with correction for my cycle by another provider).

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