How to Calculate How Far Along You Are in Pregnancy

Pregnancy duration is counted from the first day of your last menstrual period, not from the day you conceived. That distinction trips up almost everyone who tries to figure out how far along they are, because it means you’re already considered “two weeks pregnant” before conception even happens. The standard assumption is that pregnancy lasts 280 days (40 weeks) from that starting date, and everything from your due date to your trimester labels flows from that single number. But the actual calculation gets more complicated once you factor in irregular cycles, ultrasound measurements, and the reality that ovulation doesn’t always happen on schedule.

Why Pregnancy Is Dated From Your Last Period

The convention of counting from the first day of your last menstrual period (LMP) dates back to a formula attributed to Franz Naegele in the early 1800s. His original method was straightforward: take the first day of menstruation, count forward 280 days, and that gives you an estimated due date.¹ The reason clinicians still use this starting point is practical. Most people know when their last period started. Very few know the exact day they ovulated or conceived. So LMP became the universal anchor.

The 280-day figure assumes a 28-day menstrual cycle with ovulation on day 14. Under that model, actual conception happens about two weeks after the LMP. That’s why gestational age always runs roughly two weeks ahead of what’s called “conceptional age” or “fetal age.” If your doctor says you’re 8 weeks pregnant, the embryo itself is closer to 6 weeks old. This isn’t a mistake; it’s just the way the system works. Every milestone chart, trimester boundary, and due-date calculation is built around gestational age starting at LMP.

The Simple LMP Calculation

To estimate how far along you are using just your period date, count the number of days from the first day of your last menstrual period to today, then divide by seven. That gives you weeks and leftover days. If your LMP was 63 days ago, you’re 9 weeks exactly. If it was 67 days ago, you’re 9 weeks and 4 days.

For a due date, the quick version of Naegele’s rule works like this: take your LMP date, subtract three months, then add seven days and one year. If your LMP was March 10, you’d subtract three months (December 10), add seven days (December 17), and that’s your estimated due date. A large study of over 17,000 spontaneous singleton births tested this 280-day assumption and found that it holds up reasonably well as an average, though individual pregnancies vary considerably around it.²

This calculation is what most pregnancy apps and online due-date calculators use behind the scenes. You enter your LMP, and the tool counts forward 280 days. It’s a perfectly fine starting point if your cycles are regular and you’re fairly sure of the date. But it starts to wobble when those conditions aren’t met.

When LMP Dating Goes Wrong

The biggest weakness of LMP-based dating is that it assumes you ovulated on day 14 of a 28-day cycle. Many people don’t. Cycles commonly range from 21 to 35 days, and ovulation can shift significantly even within a “normal” cycle. Research comparing LMP estimates to more precise methods has found that LMP dating tends to overstate how far along a pregnancy is, most likely because ovulation often happens later than day 14.³ If you ovulated on day 21 instead of day 14, your LMP-based gestational age is already a full week ahead of reality.

Other situations make LMP dating unreliable or impossible. If you got pregnant while breastfeeding and hadn’t had a period yet, there’s no LMP to count from. The same goes for conceiving right after stopping hormonal birth control, when your first “period” may not reflect a real ovulatory cycle. Irregular bleeding early in pregnancy is another common issue: some people mistake implantation bleeding for a period and end up with a gestational age that’s off by weeks.

Ultrasound Dating and Why It’s More Accurate Early On

First-trimester ultrasound is the most reliable way to pin down gestational age. Between about 7 and 13 weeks, the embryo’s crown-rump length (the measurement from top of the head to the bottom of the torso) grows at a remarkably predictable rate that doesn’t vary much from one pregnancy to another. Ultrasound during this window can estimate the day of conception to within about five days for 95% of cases and tends to be closer than LMP dating by an average of two to three days in predicting when delivery will happen.⁴

That narrow margin of error is possible because in early pregnancy, almost all fetuses are the same size at the same gestational age. Genetics, nutrition, and other factors that cause babies to vary in size later on haven’t kicked in yet. A study comparing ultrasound-based dating to the known conception date in IVF pregnancies (where the exact day of fertilization is recorded) found that crown-rump length estimates differed from the true gestational age by less than a day on average.⁵ That’s about as good as dating gets.

The accuracy drops as pregnancy progresses. By the second trimester, ultrasound dating has an uncertainty of roughly six to seven days at 14 weeks, widening to about 12 to 14 days by 26 weeks. In the third trimester, the margin of error exceeds two weeks.⁶ By that point, fetal size reflects individual growth patterns more than gestational age, so measuring the baby tells you less about how far along you are and more about how big that particular baby is growing.

What Happens When Your LMP and Ultrasound Disagree

It’s common for LMP dating and first-trimester ultrasound to produce slightly different due dates. The American College of Obstetricians and Gynecologists has published specific guidance on how to handle this. The general principle is that once a gestational age and estimated due date are established using the best available data, they should be documented clearly and changed only in rare circumstances.⁷

In practice, the rules work roughly like this: if a first-trimester ultrasound differs from the LMP-based date by more than about five to seven days, the ultrasound date is typically used instead. In the second trimester, the discrepancy needs to be larger (roughly 10 to 14 days) before the ultrasound overrides LMP. And by the third trimester, ultrasound is so imprecise for dating that it almost never changes an established due date. The earlier the ultrasound, the more trustworthy it is for this purpose.

This matters because your gestational age affects real decisions. Whether a delivery at 37 weeks is managed as routine or treated with extra caution, whether a growth scan looks reassuring or concerning, whether labor induction is recommended at 41 weeks versus waiting: all of these depend on how far along you actually are. An inaccurate date can shift those decisions in either direction.

IVF and Known Conception Dates

If you conceived through IVF, you have something most pregnant people don’t: a known fertilization date. In IVF, the day the egg was retrieved (or the day the embryo was transferred) is documented precisely. Gestational age is then calculated by adding 14 days to the conception date, because the standard system is built around LMP, and this adjustment slots IVF pregnancies back into the same framework everyone else uses.

IVF pregnancies have been useful for research precisely because they eliminate the guesswork around ovulation timing. Studies comparing ultrasound dating to the known IVF conception date have confirmed that crown-rump length measurements in the first trimester are extremely close to the true gestational age, with differences averaging under a day for singleton pregnancies.⁵ If you’re an IVF patient, your dates are likely the most accurate of anyone’s.

What About Measuring Your Belly

Starting around 20 weeks, many prenatal providers measure the distance from the top of your pubic bone to the top of the uterus with a tape measure. This is called the symphysis-fundal height. The rough rule of thumb is that the measurement in centimeters should roughly equal the number of weeks pregnant you are: at 28 weeks, you’d expect about 28 cm.

As a dating tool, though, fundal height is poor. A systematic review and meta-analysis found that using the “1 cm equals 1 week” assumption underestimated gestational age compared to ultrasound by an average of about two weeks, with accuracy so wide that it could be off by over six weeks in either direction.⁸ Even statistical modeling using multiple serial fundal height measurements didn’t bring accuracy into a clinically useful range for dating purposes. Separate systematic review work has confirmed that ultrasound-based measurements are considerably more accurate for gestational age estimation after 20 weeks than fundal height alone.⁹

Fundal height is still useful for tracking fetal growth over time and flagging babies that may be unusually large or small, but it shouldn’t be the basis for figuring out how far along you are if ultrasound is available.¹⁰ In settings where ultrasound isn’t accessible, fundal height may be the best option, but its limitations are significant.

Home Pregnancy Tests That Estimate Weeks

Some digital home pregnancy tests now display not just “pregnant” or “not pregnant” but also a weeks estimate: typically “1–2,” “2–3,” or “3+” weeks since ovulation. These work by measuring the level of hCG (the pregnancy hormone) in urine and comparing it to threshold cutoffs that correspond to typical hormone levels at different stages. The threshold for the 1–2 week category is set at one hCG level, the 2–3 week boundary at a higher one, and so on.¹¹

These tests are surprisingly accurate for what they are. One study found that agreement between the home test’s weeks estimate and gestational age confirmed by ultrasound was about 82% when compared to the exact ultrasound date, but jumped to 98% when a clinically standard five-day margin of error was applied to the ultrasound reading.¹¹ A separate validation study reported 93% agreement between the test’s result and time since ovulation.¹²

Keep in mind that these devices give weeks since ovulation, not gestational age. To convert to the system your doctor uses, add two weeks. If the test says “2–3 weeks,” that translates to roughly 4–5 weeks of gestational age. It’s a useful early data point, but it’s no substitute for an ultrasound. It can give you a general sense while you’re waiting for your first prenatal appointment.

Why “Due Date” Is Really a “Due Window”

Even with perfect dating, only a small percentage of babies arrive on their estimated due date. The 280-day figure is an average, and normal spontaneous delivery can happen across a range of several weeks. This is part of why the medical community has moved toward thinking about “term” pregnancy not as a single block but as a spectrum. Research has shown that births at 37 to 38 weeks (“early term”) carry higher rates of complications for the newborn compared to births at 39 to 40 weeks, even though all of these fall within the traditional definition of “term.”¹³

This redefinition has practical consequences. It’s why many providers now recommend against elective induction or scheduled cesarean delivery before 39 weeks unless there’s a medical reason. When you calculate how far along you are, those fine distinctions between 37 weeks and 39 weeks are not just academic: they affect the timing recommendations your provider may give you.

Using Pregnancy Tracking Apps

Most pregnancy apps ask you to enter either your LMP or your due date, then they calculate everything from there. Behind the interface, the math is exactly the Naegele calculation described earlier. Some apps allow you to enter your ultrasound-adjusted due date instead, which is worth doing if your provider changed your dates after an early scan.

Separately, some cycle-tracking apps have begun using machine learning to predict pregnancy likelihood based on logged data about cycle length, symptoms, and timing of intercourse. One large study using data from a popular tracking app showed that models built on logged health data could meaningfully predict which users were likely to become pregnant over the following months.¹⁴ That’s a different problem than dating an existing pregnancy, but the same cycle-length data these apps collect can help you reconstruct your likely ovulation date if you’re trying to figure out how far along you are and your LMP date is uncertain.

The main thing to remember with any app is that it’s only as good as the data you feed it. If you enter the wrong LMP, the app will confidently give you wrong information for the next nine months. If your provider adjusts your due date based on ultrasound, update the app.

How Ethnicity and Body Size Affect Ultrasound Dating

First-trimester crown-rump length is remarkably consistent across populations, which is why early ultrasound dating works so well worldwide. But in the second and third trimesters, fetal measurements like head circumference, abdominal circumference, and thigh-bone length start to diverge across different populations. Population-specific reference charts have been developed for various ethnic groups precisely because a one-size-fits-all chart can introduce dating errors after the first trimester.¹⁵

The same issue applies to maternal factors. Parental height, gestational diabetes, and other conditions that affect fetal growth can make a baby measure larger or smaller than average for its gestational age. In the second and third trimesters, this means ultrasound is better understood as a growth assessment tool than a dating tool. An unexpectedly large or small measurement doesn’t necessarily mean the due date is wrong; it may mean the baby is growing faster or slower than average for other reasons. This is another reason why early dating matters so much: getting the gestational age right early prevents those later growth differences from being misinterpreted as dating errors.

Methodological Quality in Dating Charts

Not all crown-rump length charts are created equal. A systematic review of published CRL dating equations found considerable variation in how studies were designed, which populations were included, and how measurements were taken. The resulting equations produce a range of estimated gestational ages for any given CRL measurement, though using only the highest-quality studies narrows that range substantially.¹⁶ International efforts like the INTERGROWTH-21st project have since produced standardized charts based on carefully selected healthy populations, aiming to reduce this variability.¹⁷

For you as a patient, this mostly means that the specific chart your ultrasound technician uses can influence your estimated due date by a day or two. In practice, the differences between well-designed charts are small enough that they rarely change clinical decisions. But it’s worth knowing that “ultrasound dating” isn’t one monolithic system; it depends on which reference standard the machine’s software is using.

When You Have No Period Date and No Early Ultrasound

Some people arrive at their first prenatal visit well into the second or even third trimester without a known LMP and without having had an early scan. In these situations, dating becomes genuinely difficult. Second-trimester ultrasound can still provide a reasonable estimate, with uncertainty of roughly one to two weeks. Third-trimester ultrasound is even less precise, with margins of error exceeding two weeks.⁶

In these cases, providers typically use a combination of whatever information is available: approximate recall of the last period, physical examination, and ultrasound biometry. The resulting date is treated as the best available estimate, but everyone involved recognizes it’s less certain than a date anchored by a first-trimester scan. This uncertainty can complicate decisions around timing of delivery, management of preterm labor, and assessment of fetal growth, which is part of why early prenatal care is so strongly encouraged.

In low-resource settings where ultrasound isn’t available at all, fundal height and LMP recall may be the only tools. As the research shows, these are considerably less accurate, but they remain better than no estimate. For anyone in this situation, even a late ultrasound can improve on fundal-height-only dating, so accessing one when possible is worthwhile.