To estimate how far along a pregnancy is, most people use the date of the first day of the last menstrual period (LMP) and count forward about 40 weeks (280 days). If you know your conception date, you can count about 38 weeks (266 days) from that date. A medical provider can refine this with an early ultrasound, which is the most accurate single method for dating pregnancy in the first trimester. Key methods to determine gestational age
- LMP-based dating
- Start from the first day of your last period.
- Add 280 days (40 weeks) to estimate due date.
- Your current gestational age is the number of weeks since that LMP, typically expressed as “X weeks and Y days.”
- Conception date dating
- If you know the exact date of conception or have documentation (e.g., after IVF), count 266 days (38 weeks) from that date to estimate due date.
- Ultrasound dating
- Early ultrasound (weeks 9–13) measures fetal size and is highly accurate for dating, often within about ±5 days.
- Ultrasound timing becomes less precise as pregnancy advances.
Practical steps you can take today
- If you know your LMP: calculate 40 weeks from that date to estimate your due date and current week.
- If you don’t know your LMP: check with your healthcare provider for an initial ultrasound to determine gestational age.
- Track pregnancy milestones with your clinician’s guidance; regular prenatal visits help adjust dating if cycles were irregular or dates are uncertain.
Common formats for reporting
- Weeks pregnant: most people fall into a range (e.g., 6–8 weeks, 20 weeks, 37 weeks) based on LMP or ultrasound dating.
- Due date vs. gestational age: due date is the target date; gestational age is how far along, usually in weeks and days.
Important caveats
- Irregular menstrual cycles can affect LMP-based estimates.
- Inaccurate recall of the last period date can lead to miscalculation.
- If you have not had a period for a while or are unsure, rely on medical ultrasound dating for accuracy.
If you’d like, share:
- The first day of your last period (or your known conception date), and
- Whether you’ve had an ultrasound yet and, if so, approximate dating from it,
I can help you estimate your current gestational age and discuss next steps.
