How Early Pregnancy Dating Works (Before Ultrasound)
In the first few weeks of pregnancy, before you’ve had an ultrasound, your due date is almost always estimated from one thing: the first day of your last menstrual period. Here’s how that works, why it’s done that way, and what it means for you.
The standard approach
Most due dates are calculated using a rule that adds 280 days (40 weeks) to the first day of your last period. This approach has been used for a long time and is still the basis for how many healthcare providers date pregnancy before an ultrasound is done.
The last menstrual period is used as the starting point because it’s usually easier to remember than the exact day of conception. Ovulation doesn’t always happen on the same cycle day, and many people don’t track it closely. The first day of your period is a clearer reference point.
What gestational age means
When you hear “you’re 6 weeks pregnant,” that typically means six weeks from the first day of your last period—not six weeks from conception. In medical terms, pregnancy is dated from the last period, not from fertilization. That can feel counterintuitive, but it’s the standard used by the American College of Obstetricians and Gynecologists (ACOG), the National Health Service (NHS) in the UK, and many other health bodies.
This approach goes back a long way. Before ultrasound existed, the last menstrual period was the only practical reference point. Even now, with ultrasound available, LMP remains the starting point for most pregnancies until a scan provides additional information.
Why estimates can change
Early pregnancy dating is imprecise. Cycle length varies. Ovulation timing varies. If your cycles are irregular, an LMP-based estimate may be less reliable. An early ultrasound often gives a different date, and many providers will adjust the due date based on that scan if it differs from the LMP calculation.
This doesn’t mean the first estimate was “wrong”—it means early dating has natural limits. Having a range (often shown as ± several days) can help set realistic expectations.
Before your first appointment
Before you see a provider, an LMP-based estimate is usually the only information available. Tools like pregnancy calculators use the same logic: you enter the first day of your last period, and they estimate a due date and gestational age. These estimates are for informational purposes. A healthcare provider can confirm or adjust dates based on your situation and any early ultrasound.
Some calculators also show a confidence window—a range of dates rather than a single day. That reflects the real uncertainty in early dating. If your provider later gives you a different date, it doesn’t mean the calculator was wrong. It means more information became available.
In summary
Early pregnancy dating before ultrasound relies on the last menstrual period. It’s a practical standard, but it’s an estimate. Dates may change. That’s normal. If you have questions about your dates or your care, a healthcare provider is the right person to ask.