Pregnancy Calculator

1. Choose a dating method: last period, conception, IVF transfer or ultrasound.
2. Enter the date. For the last-period method, add your usual cycle length. For ultrasound, add the weeks and days from the scan report.
3. Read your due date, gestational age and trimester. Add an as-of date to look ahead, for example to plan leave.

A pregnancy due date, or estimated date of delivery (EDD), is the day a pregnancy reaches 40 weeks of gestational age. It is a planning date, not a prediction of the exact day labor starts.

Gestational age counts from the first day of the last menstrual period (LMP), about two weeks before conception. MedlinePlus defines it this way, so a person is "4 weeks pregnant" about two weeks after conceiving. Fetal age, counted from conception, runs about two weeks behind.

The due date anchors prenatal care. Screening tests, growth scans and delivery planning are all timed by gestational age. ACOG calls an accurate EDD one of the most important results of early prenatal care. For more on the counting itself, see how to calculate gestational age.

The calculator finds a pregnancy start date (gestational day 0) from your input, then adds 280 days. Each method reaches day 0 in a different way.

1. Last period. ACOG states that the 280-day rule, often called Naegele's rule, assumes a 28-day cycle with ovulation on day 14. For a longer or shorter cycle, the tool shifts the date by the difference, because ovulation moves with cycle length.
2. Conception. Conception is taken as day 14, so the due date is 280 - 14 = 266 days later.
3. IVF transfer. ACOG says to use the embryo age and transfer date: 261 days after a day-5 transfer and 263 days after a day-3 transfer.
4. Ultrasound. The scan gives a gestational age on the scan date. The tool subtracts it to find day 0, then adds 280 days.
5. Progress. Gestational age = the checked date minus day 0, shown as weeks and days.

ACOG also sets when an ultrasound should replace the period-based date. The earlier the scan, the smaller the gap that triggers a change:

The first day of a last period was March 2, 2026, and the usual cycle is 31 days, three days longer than the standard 28.

Calculation: day 0 = March 2 + 3 days = March 5, 2026. Due date = March 5 + 280 days = December 10, 2026. Estimated conception = March 5 + 14 days = March 19, 2026.

Progress on September 30, 2026: 209 days have passed, which is 29 weeks 6 days. That is week 30, the third trimester, with 71 days to go. Early term starts on November 19, and the full term window runs from December 3 to December 16, 2026.

Meaning: without the cycle adjustment, the due date would be December 7, three days earlier. An early ultrasound would confirm or replace either date.

Cycle length, recall of the period date and the timing of ovulation all move the estimate. An early ultrasound usually settles the question.

Cycle Length and Ovulation

The standard rule assumes ovulation on day 14. Womenshealth.gov notes that a normal adult cycle is 24 to 38 days, and that the time from ovulation to the next period can range from 7 to 19 days. That spread is why a cycle adjustment is only an estimate.

Remembering the Last Period

ACOG reports that only about half of women accurately recall their last period date. Bleeding in early pregnancy or recent hormonal birth control can also blur it.

Timing of the Ultrasound

ACOG calls a first-trimester scan, up to 13 weeks 6 days, the most accurate way to set gestational age. Crown-rump length then has an accuracy of about 5 to 7 days. Later scans are less precise.

IVF Embryo Age

With IVF, the transfer date and embryo age are known, so ACOG uses them instead of a period date.

Twins and Other Factors

The dating math is the same for twins, but twins and other conditions can change when delivery happens. That is a clinical question for your provider.

Each method has a different starting point and a different level of certainty.

ACOG Term Labels

ACOG classifies births from 37 weeks: early term 37 0/7 to 38 6/7, full term 39 0/7 to 40 6/7, late term 41 0/7 to 41 6/7, and postterm 42 0/7 and beyond. Earlier versions of this page called 37 to 42 weeks "full term", which does not match these labels.

After a Positive Test

Get a first estimate of the due date and current week before your first prenatal visit. The guide how many weeks pregnant am I walks through the count by hand.

Planning Work and Life

Use the as-of date to see which week you will be in on a trip date or a leave start date.

Checking a Scan Result

Enter the ultrasound age to see the due date it implies, then compare it with the period-based date using the ACOG table above.

Following the Trimesters

See what each stage brings in pregnancy by trimester, and weight guidance in pregnancy weight gain.

1. Entering the Last Day of the Period

The rule uses the first day of bleeding. Using the last day moves the due date several days late.

2. Treating the Due Date as a Deadline

The EDD is the 40-week point, and ACOG's full term window spans two weeks around it.

3. Counting Weeks from Conception

Gestational age starts at the last period, about two weeks before conception. Counting from conception makes you seem two weeks less pregnant.

4. Ignoring Cycle Length

A 35-day cycle puts the due date about a week later than the 28-day rule.

5. Using the day-3 Formula for a day-5 Embryo

The two IVF options differ by two days. Check your transfer report.

6. Replacing Your Provider's Date

Once your provider sets a due date from an ultrasound, use that date. Calculators do not see your scan.

The date math is exact, but the due date itself is an estimate built on assumptions about ovulation. An early ultrasound is the most accurate dating method, per ACOG.

It is a reasonable first estimate, but many due dates change after an early ultrasound. ACOG reports that only about half of women accurately recall their last period, and the rule assumes ovulation on day 14.

ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37 weeks to 38 weeks 6 days, and late term is 41 weeks to 41 weeks 6 days.

Gestational age counts from the first day of your last period, about two weeks before conception. So on the day of conception you are counted as roughly 2 weeks pregnant.

An ultrasound gives a better date. ACOG recommends dating by ultrasound, ideally by 13 weeks 6 days, when the period date is unsure.

Add 261 days to the transfer date for a day-5 embryo, or 263 days for a day-3 embryo. ACOG recommends this for pregnancies from IVF.

It can. ACOG changes the date when a scan up to 8 6/7 weeks differs from the period date by more than 5 days, with larger cut-offs later.

No. Dates cannot predict sex. Only prenatal tests or ultrasound can show it.

About four and a half calendar months. Months vary in length, so clinicians track pregnancy in weeks and days instead.

The dating math is the same for twins. Delivery timing in a twin pregnancy is a clinical decision, so your provider plans it.

No. The dates are calculated in your browser and nothing is sent to us. The Save button keeps a result only in this browser.

• Methods for Estimating the Due Date, Committee Opinion No. 700 (American College of Obstetricians and Gynecologists; 280-day rule, IVF 261 and 263 days, ultrasound redating table).
• Definition of Term Pregnancy, Committee Opinion No. 579 (ACOG; early, full, late and postterm).
• Gestational age (MedlinePlus, US National Library of Medicine; how gestational age is counted).
• Stages of pregnancy (Office on Women's Health, HHS; trimester weeks).
• Your menstrual cycle (Office on Women's Health, HHS; cycle length and ovulation timing).