Count the number of days since the first day of your last period and divide by 7. That's your number of weeks pregnant, since gestational age is counted from your last period, not from conception. If your own count and your doctor's don't quite match, that's common and usually explained by a few specific, normal reasons, not a sign anything is wrong.
The Simple Formula
Take the first day of your last menstrual period (LMP), count the number of days from then until today, and divide by 7. For example, 70 days since your LMP is exactly 10 weeks pregnant. A full term pregnancy runs about 40 weeks, or 280 days, from that same starting point.
If you don't know your exact LMP date, or you're working from a due date instead, there's a simpler route: subtract your due date from today's date, and compare that to the standard 280 day pregnancy length to work out roughly how far along you are. Our pregnancy due date calculator handles this directly, you can enter how many weeks pregnant you already know you are and get your estimated due date and conception date back, or go the other way and work backward from a due date you already have.
Why Your Own Calculation Might Not Match Your Doctor's
This is genuinely one of the most common sources of pregnancy dating confusion, and real people run into it constantly, doctors sometimes giving a due date or current week that doesn't match a self-calculated one, with no clear explanation given at the time. A few specific, normal reasons explain most of these mismatches:
Your cycle isn't exactly 28 days. The standard LMP formula assumes a 28 day cycle. If yours runs longer or shorter, your actual ovulation, and therefore conception, happened earlier or later than the formula assumes, which shifts the true dates without changing the simple calculation.
An ultrasound re-dated the pregnancy. Early ultrasounds measure the fetus directly and are generally considered more accurate than LMP-based counting alone, especially if your cycles are irregular or you're unsure of your exact LMP date. If an ultrasound measurement differs from your LMP based date by more than about a week, providers often update the due date to match the scan, not because something is wrong, but because direct measurement is simply more precise than calendar math.
The LMP date itself was misremembered or uncertain. This happens more often than people expect, and it's a completely understandable reason for a discrepancy, not a mistake to feel embarrassed about.
A small, several day difference between your own count and your provider's is well within normal range and isn't something to be concerned about on its own.
Calculating From Your Due Date Instead
If you already have a due date, whether from a doctor, a test, or your own earlier calculation, and want to work out your current week without re-entering your LMP, subtract 280 days from your due date to get your estimated LMP, then count forward from there the same way. This is also useful if you want to go a step further back to an estimated conception date, roughly 266 days before your due date rather than 280, since conception itself happens about two weeks into that count.
Why "Weeks Pregnant" and "Weeks Since Conception" Are Different Numbers
This is worth understanding on its own, since it explains a specific and very common point of confusion: at 6 weeks pregnant, you've typically only been pregnant, biologically, for about 4 weeks. Gestational age (what doctors use, and what "weeks pregnant" normally refers to) counts from your last period, while your baby's actual developmental age counts from conception, roughly two weeks later. Neither number is wrong, they're just measuring from two different starting points. We've covered this gap in more depth, including why it sometimes causes real confusion between partners or across appointments, in when was I conceived.
When an Ultrasound Changes Your Dates
If a scan shifts your due date by several days to a week or so, that's a normal part of refining an estimate with better information, not evidence that your original calculation or memory was wrong. Providers generally only adjust meaningfully when the scan and the LMP based estimate disagree by more than about a week, smaller differences are typically left as is. If your dates shifted more than that and nobody explained why, it's completely reasonable to ask your provider directly what measurement they used and how it compared to your LMP based estimate.
Frequently Asked Questions
Why did my due date change after my ultrasound?
Ultrasound measures your baby's actual size, which is generally more accurate than calendar based estimates, especially with an uncertain LMP or irregular cycles. A shift of several days to about a week is normal and doesn't indicate a problem.
How many weeks pregnant am I if I don't know my last period date?
An ultrasound is the most reliable alternative, since it estimates gestational age directly from measurements rather than requiring an LMP date at all. If an ultrasound isn't available yet, a known conception or ovulation date can also be used, adding roughly 2 weeks to get an equivalent LMP based week count.
Can two different due date calculations both be correct?
In a sense, yes. An LMP based estimate and a conception based estimate are both legitimate calculations, they're just using different starting points, which naturally produces results a week or two apart. This isn't a contradiction, it's the expected result of the two week gap between LMP and conception.
Why does my own calculator say a different week than my doctor's chart?
Most often because of the LMP versus conception distinction, an irregular cycle length the simple formula doesn't account for, or an ultrasound based adjustment your doctor made that a basic calculator wouldn't know about. A difference of a few days to roughly a week is common and not usually a cause for concern on its own.
Gestational age and pregnancy dating methodology per ACOG Committee Opinion #700, Methods for Estimating the Due Date.
Medical disclaimer. This article is for general educational purposes and is not a substitute for medical advice. Consult a healthcare provider for guidance specific to your situation.