How many weeks pregnant is 57 days?

How do you calculate your baby's due date?

Pregnancy is usually 40 weeks - 42 weeks long because it’s dated from the first day of your last period, not from the first day of a missed period as you may think. This means that if you do a pregnancy test on the first day of a missed period you’re already around four weeks pregnant.

How to use our pregnancy due date calculator

To find out when your baby is due, simply enter the date of your last period plus the typical length of your cycle and then simply press "calculate".

Pregnancy tends to last 37-42 weeks so it can be tricky at first to work out how many weeks pregnant you are. When your midwife or GP will work it out, they will measure your pregnancy from the start of your last period - your baby is actually two weeks younger than the start of your last period. So, whilst your doctor may tell you that you are four weeks pregnant, your baby's gestational age is actually two weeks. We hope that helps you better understand your weeks of pregnancy.

Will my baby be born on my due date?

Try not to put too much thought into the date though – unless you’ve had fertility treatment you won’t know exactly when in your cycle your egg was fertilised, so the estimated due date is just a rough guide.

How far can you pass your due date?

Only 5% of babies are actually born on their estimated due date; the majority are born two weeks either side.

Will my due date change when I have a scan?

Yes, it may do. Your first pregnancy scan will probably happen when you are 10-14 weeks pregnant; known as the dating scan. This will be used to give you a more accurate due date based on the size and the development of your baby.

Can I plan my due date?

To be honest, it's verging on the impossible when it comes to planning your due date. Many may want a summer baby or even a Christmas baby, however many women may not even fall pregnant when they want to - and only 4% of babies are born on their due date, so if you were lucky enough to fall pregnant in your chosen month, the chances of your baby being born on your due date is small.

Alternative ways to work out your pregnancy due date

Most women will ovulate and conceive two weeks after their last period and will only be around 38 gestational weeka by the time their pregnancy has reached the 40-week mark. It's thought that the dating scan is the most accurate way of determining your due date.

In order to work out the length of your menstrual cycle, you should count from the day your period normally begins to the day before your next period - the average cycle is 28 days.  In order to work this out you should take the first date of your last normal period, add 7 days to that number. Take this date, add nine months and you will get your due month. While this is another way you can work out your due date, it is not the most reliable!

Finding out how many weeks pregnant you are may seem like a straightforward question, 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 date.

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 to two weeks. 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 research 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:

1. 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.
2. 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.
3. 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.

It’s not.

In fact, that guideline about 280 days comes from Dr. Franz Naegle’s research in the early 1800s. 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 LMP.

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 sample. 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 women.

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 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.

What are the symptoms in the first few weeks of pregnancy?

 We’ve put together some week-by-week pregnancy content to let you know what to expect in those early weeks. Check out our posts on early pregnancy symptoms, where you can find out what’s going on in week one, week two, week three—and beyond.

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 42.

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 weeks. 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.

By Aarthi Gobinath, PhD | Apr 26, 2018

Aarthi Gobinath, PhD

Aarthi Gobinath earned her PhD in neuroscience from the University of British Columbia. Her research covers the ways that stress affects the male and female brain differently.

She tackled the issue of sex bias in research by looking at why standard treatments for depression don't always work in the case of postpartum depression. Her work has been covered by Vice and Massive Science.

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