How accurate is the pregnancy due date calculator?

The LMP-based calculation is accurate within 1–2 weeks for women with regular 28-day cycles, which is why it's used as a starting estimate in all clinical settings. An early first-trimester ultrasound (before 14 weeks) is more precise—accurate to within 5–7 days—and is used to confirm or revise the EDD if there is a significant discrepancy.

What if I don't know the exact date of my last period?

If you're unsure of your LMP, estimate the first day of your last period as best you can and note that your result carries more uncertainty. Your OB or midwife will order an early dating ultrasound using crown-rump length measurement, which can establish gestational age reliably without needing your LMP.

Can I use the calculator if my cycles are irregular?

Yes, but with caution—enter your best estimate of your LMP and average cycle length, but understand the result is less reliable with irregular cycles. In these cases, a first-trimester ultrasound is especially important for accurate pregnancy dating and should not be skipped.

How many weeks pregnant am I right now?

Enter the first day of your last menstrual period into the calculator, and it will compute your current gestational age in weeks and days. Gestational age is always measured from LMP, so at the moment of a positive pregnancy test, most women are already 4–5 weeks pregnant by this standard.

What is the difference between due date and conception date?

Your due date is 40 weeks from your LMP, while conception (fertilization) typically occurs about 14 days after your LMP during ovulation. This means your gestational age at any given point is approximately 2 weeks more than your fetal age—doctors always use gestational age in clinical practice.

When does each trimester start and end?

The first trimester is weeks 1–13, the second trimester is weeks 14–27, and the third trimester begins at week 28 and continues through delivery. These boundaries guide your prenatal care schedule, with different screenings, vaccinations, and developmental checks recommended in each phase.

Is a due date ever changed after the first ultrasound?

Yes, it's common for your OB to revise your EDD after an early ultrasound if the crown-rump length measurement suggests a gestational age that differs from your LMP-based estimate by more than 7 days in the first trimester or 10 days in the second. Once updated by ultrasound, the revised date is used for all subsequent care decisions.

What week is considered full term?

ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days, early term as 37–38 weeks, and late term as 41 weeks 0 days through 41 weeks 6 days. Babies born before 37 weeks are considered preterm; delivery before 39 weeks without a medical reason is generally discouraged because organ maturation—especially lung and brain development—continues through week 39.

Pregnancy & Due Date

Estimate due date, current week, trimester and key milestones from your last period.

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First day of last menstrual period (LMP)

Based on Naegele's rule (LMP + 280 days). An estimate — always confirm with your doctor.

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Estimated due date

Thu, May 27, 2027

10 weeks 0 days pregnant · First trimester

Progress25%

Weeks along

10w 0d

Trimester

First

Days to go

210

Est. conception

9/3/2026

AI Breakdown & Smart Takeaway

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How the Pregnancy & Due Date works

The Pregnancy & Due Date Calculator estimates your expected due date, current week of pregnancy, trimester, and key developmental milestones based on the first day of your last menstrual period (LMP). It's designed for anyone who is pregnant or trying to conceive and wants a clear, week-by-week picture of their pregnancy timeline.

This calculator uses the first day of your last menstrual period as its anchor point—a method standardized by the American College of Obstetricians and Gynecologists (ACOG). Even though conception typically occurs about two weeks after your LMP, the LMP date is used because most people know it reliably, whereas the exact date of ovulation or fertilization is rarely known with certainty. By adding 280 days (40 weeks) to your LMP, the tool generates your estimated due date (EDD) using Naegele's Rule, the same calculation your OB or midwife uses during your first prenatal visit.

Once your due date is established, the calculator works backward and forward to tell you exactly how many weeks and days pregnant you are right now, and which trimester you are in. The first trimester spans weeks 1–13, the second covers weeks 14–27, and the third runs from week 28 through delivery. These boundaries matter clinically: your risk profile, recommended screenings, and the types of prenatal care you receive shift meaningfully at each transition. Knowing precisely where you are in this timeline helps you prepare for appointments like the nuchal translucency ultrasound (weeks 11–14), the anatomy scan (weeks 18–22), and glucose screening (weeks 24–28).

The accuracy of any due date estimate depends on cycle regularity. Naegele's Rule assumes a standard 28-day cycle with ovulation on day 14. If your cycles are consistently longer—say, 35 days—ovulation likely occurs around day 21, meaning conception happened later than the formula assumes, and your true EDD may be about a week later. Many calculators, including this one, allow you to enter your average cycle length to adjust the estimate accordingly. For the most precise dating, an early first-trimester ultrasound (before 14 weeks) remains the gold standard, as fetal crown-rump length measurement at this stage is accurate to within 5–7 days.

A common mistake is confusing gestational age with fetal age. Gestational age counts from the LMP and is used in all clinical contexts; fetal age (also called embryonic age) counts from conception and is roughly two weeks less. When your doctor says you are '8 weeks pregnant,' they mean 8 weeks of gestational age, not 8 weeks since fertilization. Another frequent error is using an incorrect LMP date due to irregular periods or recent hormonal contraceptive use, which can throw off the estimate by a week or more. If you are unsure of your LMP, an early ultrasound is the most reliable way to establish your due date.

Formula

Estimated Due Date (EDD) = LMP Date + 280 days (Naegele's Rule). Adjusted EDD = LMP Date + 280 days + (Actual Cycle Length − 28 days). Current Gestational Age = Today's Date − LMP Date (expressed in weeks and days).

Pro tips

Enter the first day of your LMP—not the last day of your period—for an accurate due date; this single-day error is the most common reason for a miscalculated result.
If your average menstrual cycle is not 28 days, use the cycle-length adjustment field. A 35-day cycle shifts your EDD approximately 7 days later; ignoring this can misalign your prenatal screening schedule.
Screenshot or save your results and bring them to your first prenatal appointment. Your provider may update your EDD after an ultrasound, and having your LMP-based estimate helps them contextualize any discrepancy.
Use the milestone timeline output to pre-schedule critical screenings: book your nuchal translucency scan before week 14 and your anatomy scan around week 20, as availability at imaging centers can be limited.
Remember that only about 5% of babies are born exactly on their due date—full-term delivery between weeks 39 and 41 is normal. Use your EDD as a planning target, not a hard deadline.

Key terms

Last Menstrual Period (LMP): — The first day of the most recent menstrual period before pregnancy, used as the universal starting point for calculating gestational age and due date.
Estimated Due Date (EDD): — The projected date of delivery, calculated as 40 weeks (280 days) from the LMP, representing the midpoint of a normal full-term pregnancy range of 39–41 weeks.
Gestational Age: — The age of the pregnancy measured in weeks and days from the first day of the LMP, the standard clinical measure used by all healthcare providers.
Trimester: — One of three roughly equal phases of pregnancy—first (weeks 1–13), second (weeks 14–27), and third (weeks 28–40)—each characterized by distinct fetal development stages and clinical milestones.
Naegele's Rule: — The standard obstetric formula for estimating the due date by adding 280 days (or 9 months and 7 days) to the first day of the last menstrual period.
Crown-Rump Length (CRL): — The measurement from the top of the fetal head to the bottom of the torso used in early ultrasound to accurately determine gestational age when the LMP is uncertain.