Gestational Age Calculator — Weeks & Due Date

Use the Gestational Age Calculator

Estimate how many weeks pregnant you are and your due date. Supports LMP, ultrasound, conception, and IVF.

Start timing your pregnancy

How to Use Gestational Age Calculator — Weeks & Due Date

1. Step 1: Choose method

   Select LMP, Conception, Ultrasound, IVF, or Due Date.

2. Step 2: Enter dates

   Provide the required date(s); add cycle length or measured weeks if asked.

3. Step 3: Set as-of date

   Pick the date to calculate gestational age (defaults to today).

4. Step 4: Calculate

   Tap Calculate to see weeks+days, EDD, trimester, and milestones.

5. Step 5: Review & save

   Review key dates and save or share for appointments and planning.

Key Features

• Weeks+days display
• Multiple dating methods
• EDD and trimester
• Milestones timeline

Understanding Results

Formula

The calculator uses clinical conventions. Starting from the first day of your last period (LMP), the estimated due date (EDD) is calculated by adding 280 days (40 weeks). If you start from a conception date, we subtract 14 days to estimate the clinical LMP before applying the same 280-day rule. For IVF, standard offsets are used: EDD = transfer date + 261 days (day-5) or + 263 days (day-3). For ultrasound, we back-calculate LMP from the measured gestational age at the scan, then compute weeks and EDD.

Reference Ranges & Interpretation

Most full-term pregnancies deliver between 39 and 40 weeks. Early ultrasounds (first trimester) usually give the most precise dating. A small difference between methods is common and does not imply a problem. If your provider's date differs, they may be following ultrasound-based thresholds to set or revise the EDD. Use your result for planning and questions to discuss at your next visit.

Assumptions & Limitations

Results assume a single pregnancy and typical cycle physiology. Cycle length adjustments estimate ovulation timing but are less reliable with irregular cycles—prefer ultrasound dating when available. This tool is informational and does not provide medical advice. For symptoms, concerns, or conflicting dates, consult your healthcare provider.

Complete Guide: Gestational Age Calculator — Weeks & Due Date

Use our gestational age calculator to estimate weeks pregnant and your due date. Supports LMP, ultrasound, conception, IVF. Privacy-first and mobile-friendly.

This guide explains how gestational age is calculated, why multiple dating methods exist, and how to read your results with context. Whether you know your last menstrual period, have a recent ultrasound, or conceived with IVF, the tool adapts so you get an accurate weeks‑and‑days estimate and a clear estimated due date (EDD).

What is gestational age?

Gestational age is the standard way pregnancy progress is measured in clinical care. It counts the time from the first day of the last menstrual period (LMP) rather than the date of conception. This convention exists because the first day of a period is typically easier to remember than ovulation, which varies by person and cycle. When you see a value like “13 weeks and 4 days,” that is the clinical gestational age used to schedule scans, interpret test results, and plan prenatal visits.

Gestational age is not the same as fetal age. Fetal age starts around the date of conception (about two weeks after LMP in a 28‑day cycle). That is why conception‑based calculations often add 14 days when converting to clinical gestational age. Our gestational age calculator performs this conversion under the hood so your results align with medical guidelines and appointment schedules.

How this gestational age calculator works

The calculator supports five common dating methods: LMP, known conception date, ultrasound dating (enter the measured gestational age at the scan), IVF with day‑3 or day‑5 transfer, and known estimated due date (EDD). For each method, the app estimates the clinical LMP and then computes gestational age on the date you choose (today by default). It also calculates your EDD, trimester, week number, and a set of milestone dates you can save or share.

If you are exploring due dates rather than current age, our pregnancy due date calculator lets you start from the EDD directly. If you are planning conception, the ovulation calculator and conception date calculator are good companions.

LMP dating method

LMP dating is the default approach in prenatal care. Enter the first day of your last period and your average cycle length. The calculator applies Naegele’s rule—EDD equals LMP plus 280 days—and optionally fine‑tunes the result for cycle length. With longer cycles, ovulation occurs later, so due dates can shift slightly; with shorter cycles, the opposite happens. Gestational age is the number of days since the estimated LMP expressed in weeks and days.

Many people use LMP because it is the easiest to remember. If you have irregular cycles or are unsure of your dates, ultrasound dating (below) can be more accurate in early pregnancy. You can also cross‑check your trimester timing in our trimester calculator or view what happens each week with the pregnancy week calculator.

Conception date and IVF transfers

If you know the conception date, the calculator converts it to clinical gestational age by subtracting 14 days to estimate the LMP and then applying the same 280‑day rule for your EDD. This is useful for well‑timed cycles, assisted insemination, or when you are confident about ovulation timing. Our conception date calculator provides more planning details if you want to explore scenarios.

For IVF, gestational age is standardized using the embryo’s age at transfer. A day‑5 transfer uses a different offset than a day‑3 transfer. Clinically, EDD equals transfer date plus 261 days for day‑5 transfers, or plus 263 days for day‑3 transfers. Our calculator implements these medical conventions so your week count and EDD align with clinic schedules. If you also want an IVF‑specific summary, see the IVF due date calculator.

Ultrasound‑based dating

Early ultrasounds are often the most accurate way to date a pregnancy. If you have a scan report with a measured gestational age—such as “7w4d on March 1”—enter the ultrasound date and the measured age. The calculator works backward to estimate the LMP and then forward to compute your current gestational age and EDD. If your scan used a crown‑rump length (CRL) measurement, the underlying medical software performed a similar conversion from CRL to gestational age using validated charts.

In later trimesters, ultrasound dating becomes less precise because babies grow at slightly different rates. When scan‑based dates conflict with LMP dates, clinicians follow standardized rules to decide whether to revise the EDD. The American College of Obstetricians and Gynecologists summarizes those thresholds in its guidance on estimating due dates.

Adjusting for cycle length

The standard 280‑day pregnancy length assumes ovulation around day 14 of a 28‑day cycle. If your average cycle is longer or shorter, ovulation likely occurs later or earlier, which can shift your EDD. Our calculator lets you enter an average cycle length to fine‑tune the due date. This adjustment is most meaningful when you have consistent cycles and are confident about your LMP. If cycles vary widely, prefer ultrasound dating or cross‑check with the ovulation calculator.

Implantation also varies by a few days and can influence pregnancy tests and early symptoms. If you are tracking very early signs, the implantation calculator can help you estimate when implantation likely occurred based on ovulation timing.

Trimester boundaries and care scheduling

Clinically, the first trimester runs through 13 weeks and 6 days. The second trimester spans 14 weeks to 27 weeks and 6 days. The third trimester starts at 28 weeks and continues to delivery. These boundaries guide when to schedule key visits and tests—like the anatomy scan around 20 weeks and glucose screening between 24 and 28 weeks. Our calculator shows milestone dates based on your inputs so you can plan appointments with confidence.

If you want a quick view focused only on trimester windows and printable summaries, try the trimester calculator. For a week‑by‑week perspective that maps symptoms and development, check the pregnancy week calculator.

Key milestones by week

While every pregnancy is unique, certain milestones happen within typical windows. A heartbeat is commonly seen by 6–7 weeks on transvaginal ultrasound. The first trimester ends at 13w6d. The detailed anatomy scan is usually around 20 weeks. Viability thresholds discussed in prenatal care often start around 24 weeks, with care plans tailored to individual risks. Third trimester visits become more frequent as you approach term (39–40 weeks). The results panel displays your personalized dates for these touchpoints.

Growth curves, weight gain targets, and movement patterns vary. Your care team will interpret any findings in the context of your history and prenatal testing. When in doubt, ask questions—understanding your timeline helps you make informed decisions.

Interpreting your results with confidence

The gestational age calculator gives a precise week‑and‑day count for planning, but it is still an estimate. Due dates represent the middle of a natural delivery window. Most full‑term pregnancies deliver around 39–40 weeks, and many healthy births occur a little earlier or later. If your result differs from what your provider shared, it may be because they are using ultrasound dating or clinic‑specific policies on revising the EDD. Bring your numbers to your next visit to compare approaches.

Try running your dates across two methods—LMP and ultrasound, for example—to see how they align. If you conceived with IVF, use the IVF option here and cross‑validate in the IVF due date calculator. These cross‑checks help you understand small differences and pick the baseline your clinic prefers.

Small dating discrepancies are common

Calendar methods, ovulation tracking, and ultrasound each carry small, different error ranges. It’s normal for gestational age to shift by a few days when a high‑quality first‑trimester ultrasound is available—most clinics prioritize that measurement. Use the gestational age calculator to translate between methods, then follow your clinic’s lead for scheduling. Thinking in windows rather than exact days keeps expectations realistic as new information arrives.

Practical tips for using this tool

Save or screenshot your results so you can discuss them at your next appointment. If you track cycles, enter your average cycle length to refine the EDD. If the calculator shows you are overdue or early for a test, remember that clinics can adjust timing based on your history—use these dates as a planning guide, not medical advice. For a quick check by trimester, jump to the trimester calculator.

If two methods differ by a few days, anchor to the one your clinic prefers and keep the other as context. A high‑quality first‑trimester ultrasound often sets the baseline; later scans are more useful for growth than for re‑dating. Thinking in windows (for example, “around 12 weeks” rather than a single day) makes schedules and expectations easier to manage.

We do not store or share your data. All calculations happen in your browser for privacy. You remain in control of what you input and keep.

Frequently Asked Questions