How do you date a pregnancy?

pregnancyIn the first case-based class of medical school, students are asked to answer a virtual patient’s question about the development of the fetus. These students are smart and they know all about betaHcG and are anxious to showcase their knowledge of the menstrual cycle with fluctuating levels of various hormones (FSH, progesterone, etc.). Yet one question brings confusion, “How pregnant is this women?” The related question,  “When does pregnancy start?” leaves the students flummoxed. Is it at conception? But how do you know when that happens? Or does implantation make more sense? It’s a great example of how detailed facts need the larger context.

The usual dating is gestational age, based on the first day of your last menstrual period. However, you can also date a pregnancy with embryological age, starting at conception.

How you date a pregnancy can depend on your perspective. My very general guideline:

  • Pregnant woman is the focus = gestational age (e.g. obstetricians) 1
  • Focus on embryological/fetal development = embryological age (e.g. developmental biologist) 2

But why are there two types of dates? We might need a bit of a primer on the menstrual cycle and how it relates to pregnancy.

Graphic updated to remove the implantation step. Implantation happens between day 20 and 22. Pregnancy is often detected after the first missed period.

Graphic updated to remove the implantation step. Implantation happens between day 20 and 22. Pregnancy is often detected after the first missed period.

This graphic is intentionally simple, removing all the hormones and other fun stuff. You’ll note that it says approximately day 14 and day 28. In textbooks, we often see that women have 28 day cycles and everything has a nice schedule. However, women are not textbooks and sometimes have shorter or longer cycles and/or have ovulation at slightly different times. Therefore, knowing when fertilization and conception happen can be a bit tricky. An obvious marker is the first day of the last menstrual period (LMP). Why the first day? Well, another variable is the length of menses but everyone has a first day so to be consistent, that is the marker used.

We generally use gestational age when discussing pregnancy. So when someone says that they are 8 weeks pregnant, they mean it has been 8 weeks since the first day of the LMP (last menstrual period).

But that means that the first two weeks of pregnancy has nothing happening. If you are concerned about development, you don’t start counting at week 3 but start at the time of fertilization, two weeks later. Therefore, the embryological age is generally two weeks later.

pregnancy agesBut remember, we have essentially picked gestational age as the convention for discussing pregnancy dates. If  there are markers in development to suggest that the embryological age is different (for example, the fetus is 12 weeks, not 13 weeks), the gestational age is often reported to the mother. In our example, the dating would be changed to 14 weeks.

Due to the difference in these dates, we see confusion beyond medical students thinking about this for the first time. It was recently reported that Arizona had changed its abortion law to be the most restrictive – but it hadn’t. It had just joined other states in making the limit 20 weeks gestational age. Remember, this is the accepted convention for pregnancy dating – but many articles picked up on that initial two weeks of nothingness in gestational age and confused it with embryological age. Was this an example of details without understanding of the greater context?


  1. Synonyms include obstetrical and menstrual age.
  2. Synonyms include developmental, conception, and fetal age.


  1. says

    (Disclaimer: I’m neither a doctor nor a biologist.) Based on books I have read (e.g., Taking Charge of Your Fertility) and peer-reviewed journal articles I’ve read (related to whether or not levonorgestrel disrupts implantation), it is my understanding that implantation generally occurs between 7 – 10 DPO, or CD 21 – 24 (assuming ovulation and conception on CD 14). Is that understanding incorrect, or is your graphic misleading?

    • genegeek says

      That’s an excellent point. The graphic is misleading – forgot that the day 28 was still present over the ovulation point. I’ll fix it!

  2. says

    Fantastic post and perspective! Just one minor heads up: the implantation window is 6-12 days after ovulation, with the majority of implantations (>80%) occurring between 8-10 days. So the implantation stage would be day 20-22 rather than day 28.

    • genegeek says

      Thanks! I’ve tried to update the graphic and still keep it simple. The day 28 thing over ovulation was a definite mistake (originally had a day 21 column). I hope the update makes some sense.

  3. genegeek says

    Thank you for the comments! I have removed the implantation step from the graphic because it was a detail that wasn’t crucial to the main point of this post – although very important for the process.

  4. Michelle says

    “It was recently reported that Arizona had changed its abortion law to be the most restrictive – but it hadn’t. It had just joined other states in making the limit 20 weeks gestational age.”

    This is incorrect, what obstetricians do is use Naegels rule to estimate the age of the pregnancy, this is a method of dating the pregnancy not of defining it. When pregnancy biologically starts is a red herring, the woman is known to be pregnant, it’s the approximate due date that is needed. This is and was never meant to be an accurate estimation of gestational age of the foetus, and is known to need adjusting should a woman’s cycle be shorter or longer than average. This also ignores that since the development of ultrasound, there is more accurate dating of actual gestational age of the foetus, which is also used to provide information as the length of the pregnancy. The EDD given on ultrasound is often different.

    The Arizona legislature used this method of dating as a definition, deliberately ignoring that they should have used gestational age as the definition. They knew full well this means that the pregnancy is actually two weeks less than that adjusted (doctors count 40 weeks LMP, 38 weeks gestational age) and this means if a woman gets the anatomical scan at around 20 weeks, if a foetal abnormality is detected, she and her doctor can’t do a thing about it even though the gestational age of the foetus is actually 18 weeks and they could have two weeks to make a decision. This had to be deliberate, the vast majority of terminations occur much earlier and where it happens later it is normally for medical reason, related to the foetus or mother’s health which was information available to them. It also knowingly created an anomaly where the woman is considered pregnant before she actually is, two or so weeks before conception and implantation of a viable embryo could occur.

    There appears to be some misinformation put about that doctors use this, therefore this legislation is just fine. This is very obviously incorrect but people are buying it. This is just another step in making terminations of pregnancy unavailable.

    • genegeek says

      I may not be following your explanation but the point of the post is that there are two ways to date a pregnancy. Gestational refers to dating from the LMP and developmental refers to conception or what is seen on the ultrasound. If a different date is seen on an ultrasound (i.e. fetus was expected to be at 15 weeks but is only at 14 weeks), the gestational age is adjusted (in the bracket example, gestational age would be moved to 16 weeks).
      If a pregnancy was dated by ultrasound to be 20 weeks developmental as used in your example, the gestational age would be 22 weeks. If the legislation was referring to developmental or embryological age, it would allow an extra two weeks for abortions compared to other jurisdictions using gestational age.

  5. Michelle says

    Genegeek, sorry if my explanation was confusing, I think I may have used gestational age when I should have stated foetal age there? In any case, the Arizona legislature put into law the gestational age (based on LMP) not foetal age. Adjustments made on more accurate dating via ultrasound is not part of the definition, nor does it account for that the LMP method counts 40 weeks elapsed pregnancy, whereas foetal gestation would count the pregnancy to be the true 38 weeks. Effectively, using that definition means an termination of pregnancy ban from 18 weeks, assuming normal 28 day cycle and I’m sure those politicians knew that. It also affects non-surgical terminations, which need to be done well before 12 weeks as the woman will be two weeks further along.

    That makes this incorrect “It was recently reported that Arizona had changed its abortion law to be the most restrictive – but it hadn’t. It had just joined other states in making the limit 20 weeks gestational age.” I’ve seen others say this, using that medicine uses LMP to determine length of pregnancy as an excuse to justify this. Other states definitions actually use conception – i.e. Georgia “probable gestational age is an estimate made to assume the closest time to which the fertilization of a human ovum occurred…” In contrast, the Arizona law says “probable gestational age means the gestational age of the unborn child at the time the abortion is planned to be performed…”

    The reason why doctors use LMP is that it’s almost impossible to precisely identify when ovulation occured, but with LMP a woman is likely to remember at least roughly the correct date. It’s got nothing to do with defining when pregnancy started, but rather is a means of estimating length of gestation and the likely due date which has some very obvious clinical ramifications, say if premature labour occurs. This is not meant to be used as any sort of legal definition of anything, it’s just a way of calculating the length using a general rule. Maybe this explains it better:

    “Pregnancy can be measured in two different ways. The most common and most often used in clinical obstetrics is the measurement of the gestational age of the pregnancy. The gestational age of the pregnancy is measured from the first day of the last menstrual period. In this way of dating the pregnancy, the pregnancy is 40 weeks in duration (on average) instead of the actual 38 weeks. In other words, it dates the pregnancy, on average, two weeks longer than it is.

    The other way of measuring the dates of the pregnancy is to measure the fetal age. The fetal age of the pregnancy is measured from the time of conception or the estimated time of conception (ETC). When measuring the pregnancy in this fashion, it will be 38 weeks long or two weeks shorter than the gestational age dates.

    *The fetal age, of course, is the actual age of the pregnancy.*”

    Note that the ETC/foetal age method, and/or using ultrasound will both give an accurate dating of the age of the pregnancy and the foetus, the other will be out by two weeks, or more if a woman has a long cycle.

    • genegeek says

      Hi Michelle. I think we are circling around the same things. I agree with what you said about dating pregnancy – your quotes are what I was trying to say in the post.
      In relation to Arizona being two weeks earlier, we will have to agree to disagree. In every state and country that I’ve worked in, the date of a pregnancy used for terminations is gestational. If there is better dating from an ultrasound, the gestational age is adjusted. (I have never worked in Georgia – but if they are using gestational age in the definition, it means from the LMP, perhaps adjusted due to ultrasound findings.)
      There are MANY things wrong with Arizona’s recent laws re: women. In my experience, the dating isn’t more stringent but some of the other language and inclusions are. For example, including the non-fact that abortion leads to psychological damage. Dr. Jen Gunter does a better job with this part:

  6. Darci says

    At the risk of sounding completely uneducated, here it goes:

    My last cycle started Feb 9, my cycle is 31 days, I had what appeared to be implantation bleeding Feb 21-22, and I am now two days late. However, I am still trying to calculate the date of conception. Can anyone help clarify for me? Thanks!

    • says

      Implantation happens 7-10 days after conception, which would mean that conception would have taken place between the 11th and 14th. Just 2-5 days after start of menses? That seems rather unlikely, but as a mere amateur I can’t say it’s impossible.

      Pardon the intrusive question, but when did you have intercourse? The bleeding may only be from that. Also, is your cycle always 31 days, or is that just an average? Even if the bleeding was just a side effect of intercourse (or something else not related to implantation), you might still be pregnant. Two days late is iffy as evidence goes. Assuming a 14-day luteal phase, and a 31-day cycle, that’s just 16 days past ovulation. Fertility awareness methods (such as found in the excellent book “Taking Charge of Your Fertility”) state that pregnancy is highly likely once 18 days past ovulation is reached. Technically, they refer to 18 high temperatures, but you haven’t mentioned tracking that.

      If you’re not pregnant, it’s possible that something caused ovulation to be delayed. That wouldn’t change the length of the luteal phase, though, so your cycle would just be longer by however many days ovulation was delayed. Stress can delay ovulation. Have you been under any stress lately? Move? Change jobs? Something else?

      I wouldn’t be surprised if there’s something I missed, but those are the possibilities that come to the mind of this amateur. :)

  7. genegeek says

    In general, the second part of the menstrual cycle (luteal phase = between ovulation and menses) is fixed at 14 days. So you should have ovulated around 17 days after Feb. 9 (or 26th). But this is an average so you may be different – and that bleeding on 21/22 may have been implantation. How’s that for a non-answer?
    I wish I could give you something definite but most dating is estimates, based on averages seen for women in general. If you want to share more information that you don’t want to leave in the comments, you can always use the contact form (in the footer).

    • genegeek says

      The scans are usually pretty accurate. Sometimes women don’t realize that they are pregnant because the first missed period is just lighter instead of missing – so they are 4 weeks more pregnant than they thought. (I hope that makes sense)

  8. shazzz says

    I also have a problem in counting my due date my LMP was on the 30 of November an due on the 7 of September but I did ultrasound in may the19th an gave me a different EDD of which it was 21of September my question is when does an ultrasound start counting pregnancy ,is it on from fertilization or ovulation

    • genegeek says

      The ultrasounds usually look at markers of fetal growth and other aspects of pregnancy and then report using the ages mentioned in this post. Many people use the ultrasound to better estimate their due dates because of the confusion that comes with varying lengths of menstrual cycles.

Any comments? Please play nice.