Narrowing down conception date

March 12, 2014 4:41 PM

Is it possible to narrow down conception date to less than a week based on HCG levels?

Relevant information:
1. Menstrual period is typically irregular
1. Last date of menstrual period: January 31st
2. Had sex on Feb 13th (&14th) and Feb 19th (the dates to be distinguished)
3. Ovulation was expected to be on the 13th but as I mentioned, period is irregular.
4. HCG level measurements on 3/7 at 3pm was 1000 mIU/ml
5. HCG level measurements on 3/10 at 2pm was 2800 mIU/ml
6. Ultrasound was performed today 3/12 - can't quite see the sac yet.

Judging from LMP, 3/7 should be 5th week but HCG level seems too low (5th week level should be 2,580-6,530). It is in the range of 4 weeks + 3 or 4 days which seems to indicate that conception date was sometimes around Feb 17th?

I guess the crux of the question is - is it possible at all based on those numbers that conception date be Feb 19th? Can we tell? Not enough information?

posted by anonymous to Health & Fitness (10 answers total)

Probably not enough information. If this is, like, a sixth pregnancy, and you have a good data about HCG levels from all the earlier pregnancies, and it's highly consistent, you could make an educated guess. But really, both 2/14 and 2/19 are "around Feb 17th."

(I don't know much about HCG and conception dates but I do know that individual variation in medicine usually vastly overshadows "usual" or "group" variation.)
posted by mskyle at 4:50 PM on March 12, 2014

IIRC, HCG begins rising with implantation, not conception, which can happen anywhere between 6 and 10 days of fertilization. Because of this, it's really pretty impossible to tell which day you conceived.

I wouldn't worry too much yet. It's really really early.
posted by PhoBWanKenobi at 5:00 PM on March 12, 2014

Conception happens when ovulation happens, even if that's several days after sex. So yes, it's entirely possible to have conceived on the 17th, as well as the 14th or the 19th or really any of the days in between. hCG levels vary too much to really pin it down more specifically.
posted by Catseye at 5:23 PM on March 12, 2014

If you're concerned about this because of who you slept with on which night, you can get a prenatal paternity test later on but as far as I know you need at least the potential father's DNA as well as the mother's and the baby's DNA. Otherwise, no, the date of conception can't be that granular by current methods, sorry, unless you tracked your exact dates of baby-making activities.
posted by goo at 6:14 PM on March 12, 2014

Speaking only from my own pregnancy, the time at which I was definitely pregnant on ultrasound but during which a sac couldn't really be distinguished was 4w4d - 5w6d. (I had some early ultrasounds because my hcg levels were way out of whack and we needed to rule out a serious problem.) That's measured from putative LMP, assuming that ovulation is on day 14, so they really represent 2w4d and 3w6d post ovulation, respectively. Unfortunately, in your case, that covers both the dates in question. And that's even assuming you can extrapolate from my pregnancy, which. . . you really can't.

If you check BetaBase,, you'll see that the median hCG level reported on 17DPO (that would be your 19th, dated from the 3/7 draw) is 302, while the median level for 23DPO (the 13th) is 2511. You'll also see that the numbers vary UNBELIEVABLY WILDLY; in my own example, my numbers for 22DPO were about 13,000. But even judging by the medians, the spread is too wide to pin it down.

Finally, conception is likely on the three days before ovulation, the O date itself, and the day afterwards. It's possible for an additional day or so on either side of that. So if you ovulated on, say, the 17th? you could have caught the egg with any of those episodes of intercourse, and the numbers would be identical.

All that is a very long-winded, possibility-chasing way of saying no. No, I'm sorry, there's no way to tell by your hCG numbers which of those two conception dates is more likely. They're just too close together.
posted by KathrynT at 6:25 PM on March 12, 2014

I think the answer is no/not enough info.

You really want to go to the library or bookstore and get Taking Charge of Your Fertility... it covers a lot of what you're asking.

As mentioned, even if you can pinpoint ovulation, fertilization and implantation can both take time (up to a week for the latter). If conception and HCG production begin at implantation (not ovulation)... it's gonna be hard to know. Plus, some women naturally produce lots and lots of HCG, others less. This is why they date everything back to the first day of your last period, and partly why so few babies come on their 'due date'.

I'll also note than when they did my early dating scans, they went off fetus size - not HCG levels. The ultrasound tech said that in early days size is quite accurate as the cells divide at a very consistent rate. However, this is still dating based on LMP.
posted by jrobin276 at 6:28 PM on March 12, 2014

When I learned I was pregnant I was quite far along and had no idea of the date of conception (I had bleeding in early pregnancy that I thought was menstrual). Dating went by scans, not HCG levels, and it was extremely easy to pinpoint based on scan and my temperatures. In the future, consider charting your temp (takes less than 30 seconds a day) to make this easy to track.
posted by peanut_mcgillicuty at 6:38 PM on March 12, 2014

When the embryo becomes visible, your doctor should be able to give you a pretty accurate estimate based on its size. Mine first became visible around 6 weeks, and the dates I was given then were exactly what I had thought (before that I had to do a bit of guesswork too). As you get further along, sizes start to vary more so dates become less reliable.
posted by loveatfirstsite at 11:04 PM on March 12, 2014

If you're asking this because you are concerned about your betas/ultrasounds given how far along you "should be," I'm sorry, but there's no way to backtrack that accurately unless you were charting in the first place.

If you're concerned about when you conceived from a practical purpose, same answer applies.

In both cases, time (and a short amount of time at that) will tell. As soon as the embryo is (or should be) measurable, you'll have a very accurate answer. Unfortunately this doesn't help if you're stressing about potential viability right now, hopefully you have another ultrasound/blood draw scheduled ASAP.
posted by lydhre at 12:37 AM on March 13, 2014

For added perspective on why this is impossible, check out the Betabase.

Here are the stats for 22 days post ovulation beta levels... And that's for people who actually know when they ovulated....
posted by treehorn+bunny at 1:22 AM on March 13, 2014