Pregnancy Due Date Calculator

Quick Answer

The standard last-period method estimates a due date by counting 280 days from the first day of the last menstrual period. That is Naegele’s rule: a 40-week pregnancy dated from LMP, not from conception. If your average cycle is not 28 days, this calculator adds or subtracts the difference before showing the estimated due date (EDD).

When you already know a conception date, IVF transfer, ultrasound gestational age, or a clinician-assigned due date, switch modes. You will also see how many weeks pregnant you are, which trimester you are in, and how many days remain. An EDD is a planning estimate — most births happen in a window around that date, not on it.

How to Use the Pregnancy Due Date Calculator

1. Choose the dating method you know: last period, conception, IVF transfer, ultrasound, or a known due date.
2. Enter the required date. For last period, use the first day bleeding started.
3. Add your average cycle length when using last-period mode (default 28 days).
4. Select Calculate, or read the result as soon as the date is valid.
5. Review the estimated due date, pregnancy week, trimester, and timeline.

How Is a Pregnancy Due Date Calculated?

Pregnancy is conventionally dated from the first day of the LMP even though conception usually happens about two weeks later. That is why “how many weeks pregnant am I” often sounds two weeks ahead of fetal age. The calculator never claims an exact delivery day.

Using Your Last Menstrual Period

EDD = LMP + 280 days + (cycle length − 28). A 28-day cycle uses a plain 280-day count. A longer cycle moves the estimate later; a shorter cycle moves it earlier. Cycle length is a planning adjustment, not proof of the ovulation day.

Using Your Conception Date

EDD = conception date + 266 days. The equivalent LMP is 14 days earlier. Use this only when timing is reasonably known. Outside that, last-period or ultrasound dating is usually more practical.

Using IVF Transfer Date

EDD = transfer date + (266 − embryo age). Day 3 is +263 days, day 5 is +261, day 6 is +260. Embryo age and transfer date replace an LMP assumption. Your clinic’s documented EDD still governs prenatal care.

Using an Ultrasound Result

EDD = scan date + remaining days to 40 weeks, using the weeks and days printed on the report. This is arithmetic from a provided measurement. It does not interpret images. First-trimester ultrasound is the most accurate common method for establishing or confirming gestational age when clinically appropriate.

Using a Known Due Date

Enter the EDD you were given and, optionally, a reference date. The tool counts backward to gestational age, trimester, estimated LMP, and estimated conception. That answers “how far along am I” when the due date is already set.

Pregnancy Timeline and Key Dates

The result panel lists last period, estimated conception, trimester start dates, and the EDD. Trimesters here follow common obstetric ranges: first 0w0d–13w6d, second 14w0d–27w6d, third 28w0d onward. Full-term pregnancy is usually described as 39 weeks 0 days through 40 weeks 6 days; 37 weeks is early term, not “late.”

Gestational age counts from LMP. Fetal age starts at conception and is typically about two weeks less. The result panel shows both, plus 16w3d-style clinical notation. Reports and prenatal visits use gestational age. Time remaining is counted from today (or your reference date) to the estimated due date — not a prediction of labor. If you do not know your last period, skip LMP mode and use conception, IVF, ultrasound, or a known due date instead of guessing. If you are still planning around ovulation rather than an established pregnancy, use the ovulation calculator.

ACOG describes term categories used for planning language, not as a promise of when labor starts: early term 37w0d–38w6d, full term 39w0d–40w6d, late term 41w0d–41w6d, and post-term from 42w0d. Most births occur from 37 weeks through 41 weeks 6 days. The calculator lists those start dates and a mid-pregnancy 18w–22w window often used when scheduling an anatomy scan. Those rows are calendar arithmetic from the dating start, not appointment bookings.

Due Date Calculation Examples

These rows use the same date-only functions as the calculator.

A 35-day cycle does not change conception biology by itself; it changes the LMP-based count because ovulation is assumed later in a longer cycle. Equivalent LMP for the conception example is 2026-01-01.

How Accurate Is a Pregnancy Due Date Calculator?

A due date is an estimate, not a guaranteed birth date. Only a small share of babies arrive on the exact EDD. A standard LMP calculation assumes a typical 28-day cycle and reliable recall of the first day of bleeding. Irregular cycles, late ovulation, and uncertain LMP dates all shift LMP-based estimates.

First-trimester ultrasound can provide a more accurate clinical estimate when dates conflict. IVF dating uses embryo age and transfer date, which removes the usual LMP guess — it still does not predict the hour of labor. The due date documented by your healthcare professional should take precedence for prenatal care.

Use this calculator for an estimate and planning. Follow the due date provided by your healthcare professional for clinical care. Authoritative overviews include ACOG methods for estimating the due date, the NHS due date guide, Mayo Clinic pregnancy dating, and Cleveland Clinic fetal development.

Common Due Date Calculation Mistakes

• Using the last day of bleeding instead of the first day of the period.
• Treating a conception date as if it were the LMP (that skips about two weeks).
• Assuming every cycle is 28 days when yours is consistently longer or shorter.
• Entering the wrong IVF embryo age (day 3 vs day 5 vs day 6).
• Confusing gestational age with fetal age.
• Treating an estimated due date as a guaranteed birth date.
• Ignoring an updated clinician or first-trimester ultrasound due date.

Do not use this page to confirm a pregnancy, choose prenatal tests, or decide whether to induce labor. Do not invent an LMP just to force a number. Copy link shares the dates you typed in the URL; send it only to people you trust. If you need to add 280 days on a calendar by hand, the date calculator can add days to a date. It does not apply pregnancy dating rules.

Frequently Asked Questions

How is a pregnancy due date calculated?

The usual LMP method adds 280 days to the first day of the last period, then adjusts if cycle length is not 28 days. Conception dating adds 266 days. IVF dating adds 266 minus the embryo age at transfer. Ultrasound dating adds the remaining days to 40 weeks from the gestational age on the report.

How many weeks pregnant am I?

Gestational age is the number of weeks and days from the dating start — usually the LMP. At conception you are already about 2 weeks pregnant on that scale. The result panel shows weeks + days on the reference date (today, unless you set another date).

Does cycle length affect my due date?

Yes, in LMP mode. The calculator adds (cycle length − 28) days to the 280-day count. A 35-day cycle moves the EDD 7 days later than a 28-day cycle with the same LMP. That assumes later ovulation, which is a planning model, not a lab result.

Can I calculate my due date from conception?

Yes. Enter the conception date to add 266 days. The equivalent LMP is 14 days earlier. Conception timing is often uncertain unless ovulation or intercourse timing was clear, so treat this as an estimate.

Can I calculate my due date after IVF?

Yes. Enter the embryo transfer date and whether the embryo was day 3, day 5, or day 6. The calculator uses transfer + (266 − embryo age). Your clinic’s documented EDD still governs prenatal appointments.

Can an ultrasound change my due date?

Clinicians may revise an LMP-based EDD when an early ultrasound differs by a set number of days. This tool can repeat the arithmetic from a report’s gestational age. It does not decide whether a clinical due date should change.

How accurate is a pregnancy due date calculator?

It is a planning estimate. Few babies are born on the exact EDD. LMP recall, cycle length, and ovulation timing all matter. First-trimester ultrasound is generally more accurate for confirming gestational age. Use your clinician’s date for care.

What is the difference between gestational age and fetal age?

Gestational age starts at the LMP. Fetal age starts at conception and is usually about two weeks less. Medical records use gestational age. This calculator reports gestational age unless you are in conception mode, which still converts to that scale.

What should I do if I don't know my last period date?

Use conception, IVF, ultrasound, or known-due-date mode if you have any of those. If you have none, a clinician can date the pregnancy with examination and ultrasound. Do not guess an LMP just to force a number.

What are the three pregnancy trimesters?

First trimester is 0 weeks 0 days through 13 weeks 6 days. Second is 14 weeks 0 days through 27 weeks 6 days. Third starts at 28 weeks 0 days. These ranges match common ACOG-style obstetric language, not a 1–12 / 13–26 split.

When do most babies arrive, and what is full term?

Most births occur from 37 weeks 0 days through 41 weeks 6 days. ACOG calls 37w0d–38w6d early term, 39w0d–40w6d full term, 41w0d–41w6d late term, and 42w0d onward post-term. The estimated due date is the 40-week mark. It is a planning date, not a guaranteed day of labor.