How many weeks pregnant am I? This might seem like a straightforward question to ask your doctor, but due date calculation is often inaccurate. In fact, only 4% of women actually give birth on their due dates, with a majority giving birth between one and four days after the calculated due date. Even more surprising, a whopping 30% of women don’t give birth within the two week range of their due date1.
It’s tempting to brush off a few days as a simple rounding error, but here’s why you should care: despite their inaccurate calculations, due dates are still used as hard medical deadlines. Past your due date? Your doctor will probably want to induce you after one week, or two weeks at most. More than three weeks early? You and your baby will be considered pre-term, which has major health implications.
This post will cover:
- When does pregnancy really begin? It’s probably not when you think.
- How are due dates typically calculated by doctors, and why is this method technically wrong?
- What’s the little-known secret for accurately dating your pregnancy?
- Why is it important to get an accurate due date?
When exactly does pregnancy begin?
Most people think that pregnancy starts at conception. Politics aside, that’s not entirely accurate. Recent research2 indicates that conception itself is not a rare event, but only a small portion of fertilized eggs are “fit” enough to implant. By that logic, the most accurate way of dating pregnancy would be beginning from implantation.
The standard method of dating pregnancies does not start from conception or from implantation. Rather, it starts from the first day of your last menstrual period. That’s right—your doctor considers day one of your pregnancy as the first day of your period, when you could not have been pregnant!
How accurately can medicine determine how many weeks pregnant a woman is?
Here is an explanation of the three possible start dates that could be used for determining how many weeks pregnant you are, ranked in order of accuracy:
- Most accurate is when implantation occurs. Implantation typically occurs between eight – 10 days post-ovulation (DPO). As soon as this process is complete, the embryo begins producing the hormone hCG, which should be detectable as soon as you can take a pregnancy test by around 12 DPO. The downside of dating pregnancy from implantation is that it’s very rare that you would be able to know exactly when implantation occurred.
- Less accurate is when ovulation occurs. It makes more sense to date pregnancy from implantation, but if you could know with absolute certainty when you ovulated, you’d only be off by a few days, since implantation occurs in a narrowly defined time frame after ovulation. The problem with this method is that most women don’t track ovulation, and even those who do may not be completely accurate in their estimations.
- Least accurate (and unfortunately, most common) is dating pregnancy from your last menstrual period (LMP). For any woman who doesn’t have a textbook 28-day cycle with ovulation on day 14, this method produces an inaccurate due date. Why is the least precise method for dating pregnancy also the most common one? Because it’s the easiest: way more women remember when their last period was than when they last ovulated.
How is a due date calculated?
Due dates are calculated by adding 280 days (nine months and a week) to the first day of LMP, presumably more women remember when they last had their period than when they ovulated (or even better, when implantation would have occurred). While LMP is a more accessible date, it is problematic because it inaccurately estimates how many weeks pregnant you actually are.
Can an ultrasound determine a more accurate due date?
Your doctor may adjust your due date during a “dating ultrasound,” which typically occurs during one of your first appointments. Since healthy embryos grow at a highly predictable rate during the first trimester, measuring the length of the embryo (called the “crown-to-rump” measurement) can provide a more accurate estimation of how many weeks pregnant you are.
If your dating ultrasound does not match the due date based on your LMP, your doctor may adjust your due date according to the ultrasound, as this is typically considered more accurate.
Why is dating from last menstrual period or LMP inaccurate?
Say you have a 28-day cycle, and after the two-week wait to take a pregnancy test, you got a positive pregnancy test on the day you missed your period. Your last period was 28 days ago, so even though implantation happened within the past few days, you’re already considered four weeks pregnant.
The problem with this method, aside from the fact that it considers pregnancy as beginning from several weeks before you are actually pregnant, is that it is inaccurate for any woman who does not ovulate on day 14. If you ovulated later than day 14, your due date will be set too early. And if you ovulated earlier than day 14, your due date will be set too late. The further away from day 14 you ovulate, the less accurate your due date will be.
Where did this due date calculation even come from?
You would think the 280-day calculation was based on a large statistical study analyzing how many weeks pregnant women usually are before natural labor, and based on that data, researchers found that 280 days is the average length of pregnancy.
In fact, that guideline about 280 days comes from Dr. Franz Naegle’s research in the early 1800s3. He based his observations on the 28-day cycle and found that a majority of his female patients gave birth about one week and nine months after LMP (note: he did not specify at the beginning or end of LMP, just LMP. Current convention is to go with beginning of LMP).
Despite the fact that this definition is unclear, outdated, and based on the incorrect assumption that all women have 28-day cycles with ovulation on day 14, it is still used in current medical practice.
How long are women actually pregnant for?
A study from 1990 of over 1000 women found that the median gestation length (so 50% of women) for first-time mothers was 277 – 288 days since LMP. For second-time mothers, median gestation length was slightly shorter and about 275 – 287 days since LMP4.
Interestingly, one study of a small population of 125 Caucasian women found that gestation lasted on average 268 days from ovulation, and that despite no labor or preterm complications in the sample pool, gestation length varied by 37 days across the sample5. Progesterone may explain the variability because a slower rise in progesterone after implantation correlated with a longer pregnancy. But, keep in mind, this is a small sample size with one racial background.
Although research doesn’t understand why yet, racial background does affect gestation length. A study of over 100,000 women in London found that women of African or Asian backgrounds had a median gestation length of 273 days, which is about a week shorter than Caucasian women6.
Bottom line: how many weeks pregnant the average woman is when she gives birth does vary considerably, but about 282 days from LMP seems to be more accurate 7 although whether that’s true for women of all ethnic backgrounds definitely requires more research.
I know when I ovulated, how can I accurately calculate my due date?
If you know when you ovulated and know that your cycles are longer or shorter than 28 days, tell your doctor when you ovulated, not LMP. They can work it out from there and provide a more accurate due date.
I don’t know when I ovulated. Is there still a way to get a more accurate due date?
Yes: you can get a more accurate due date with a dating ultrasound, which typically happens at one of your first pregnancy appointments. Since embryos grow at a very predictable rate, an early ultrasound is a highly accurate way of measuring how many weeks pregnant you truly are.
If your LMP-estimated due date is very different from your dating scan-estimated due date, your doctor may express concern. Let’s say you ovulated on day 25 of your cycle—about a week and a half later than the “textbook” day 14. If your doctor schedules your first ultrasound for six weeks after your LMP, it may be too early to even see a heartbeat. Based on when you ovulated, this is normal and not cause for concern: your embryo is simply a week and a half younger than someone who ovulated on day 14 of her cycle.
As long as your pregnancy continues to progress at a normal rate, your doctor will probably adjust your due date based on the dating ultrasound.
How late is too late after my due date?
This is why the question of “how many weeks pregnant am I?” is important to address. Basing due dates on LMP increases the risk of categorizing mothers as “late” or “post-term.” In these cases, your doctor will talk to you about inducing labor.
For truly post-term pregnancies, labor induction can be a safe and reasonable option. The primary cause for concern for the infant is neonatal mortality, as the risk for stillbirth sharply increases after week 42 of pregnancy. For mothers, risk of chorioamnionitis (infection of the amniotic fluid), obstetric complications, and maternal mortality increase after week 428.
What are the risks of labor induction if I’m not post-term?
Induction is often the safest option for post-term pregnancies, but it may not make sense to discuss induction until you are confident that you are truly post-term. This is because the drugs used for labor induction (typically a form of oxytocin or prostaglandins) do carry some risks. They can cause abnormal contractions, which can adversely affect the baby’s oxygen intake and heart rate, and can increase the likelihood of post-birth hemorrhage and infection. Also, there’s always the chance that the induction fails, in which case a c-section will likely be the next step.
In the case of true post-term cases, the benefits generally outweigh risks, but it’s important to be aware of this cost-benefit assessment.
What does it mean if I give birth before my due date?
Pre-term birth is defined as birth occurring before week 37, but as you can suspect, inaccurate dating can also be problematic here. There is a diagnostic range of pre-term, with very pre-term being less than 28 weeks and late pre-term being closer to 36 weeks9. Based on this spectrum, your doctor can attempt to delay delivery to help the baby grow longer in utero, or if it cannot be delayed, your baby will have to be closely monitored to ensure its health.
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3777570/ ↩
- https://www.nature.com/articles/nbt.1686 ↩
- https://obgyn.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1471-0528.2000.tb11661.x ↩
- https://academic.oup.com/humrep/article/16/7/1497/693431 ↩
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3777570/ ↩
- https://academic.oup.com/ije/article/33/1/107/668109 ↩
- https://www.ncbi.nlm.nih.gov/pubmed/10461334 ↩
- https://emedicine.medscape.com/article/261369-overview ↩
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139808/ ↩