How to Calculate Your Due Date – Three Methods Explained

The moment a pregnancy test shows a positive result, one question takes over everything else: when is my baby coming? It is the number that shapes the next nine months – when you tell your employer, when you book your first scan, when you stop travelling, when you pack the hospital bag.

Most women are given a due date at their first prenatal appointment, but that appointment can feel weeks away when you are holding a positive test at seven in the morning.

Knowing how to calculate your due date yourself, right now, is more than just satisfying – it gives you an accurate starting point for everything that follows. This guide walks through all three methods, shows you the math, and explains which approach works best depending on what information you have.

What Is a Due Date – and How Accurate Is It?

Before getting into the calculations, it helps to be honest about what a due date actually represents. Your estimated due date (EDD) is the midpoint of a normal delivery window – not the day your baby will definitely arrive. A full-term pregnancy is anywhere between 37 and 42 weeks, and according to the American College of Obstetricians and Gynecologists (ACOG), only about 4% of babies are born on their exact due date.

What the due date gives you is a reliable reference point. Your OB-GYN, midwife, and care team will use it to schedule prenatal appointments, interpret ultrasound measurements, plan genetic screening timelines, and monitor your pregnancy’s progress. An accurate due date from the start means better-timed care throughout.

The earlier a due date is established – ideally before 14 weeks – the more reliable it is. An early ultrasound, particularly one conducted between weeks 8 and 12, can confirm or adjust your calculated due date based on actual fetal measurements. If your ultrasound date and calculated date differ by more than seven days, your provider will typically adjust your official EDD to match the scan.

Use CycleWhisper’s free pregnancy due date calculator to get your estimated due date instantly using any of the three methods below.

Method 1: Last Menstrual Period (LMP) – The Most Common Approach

The LMP method is how the majority of due dates are calculated in clinical practice, and it is the approach your OB-GYN will use at your first appointment if you do not have an early ultrasound to reference.

The formula: Add 280 days (40 weeks) to the first day of your last menstrual period.

This works because pregnancy is measured from the start of your last period, not from conception. The convention exists because the start of your period is a known, documentable date – whereas the exact day of conception usually is not. It means that by the time a pregnancy test turns positive (typically around four weeks after your LMP), you are already considered four weeks pregnant in clinical terms.

A worked example:

• Last period started: January 15th
• Add 280 days
• Estimated due date: October 22nd

You can also use Naegele’s Rule, the traditional clinical shorthand: take the first day of your LMP, subtract three months, and add seven days.

• LMP: January 15th
• Subtract 3 months: October 15th
• Add 7 days: October 22nd

Both approaches produce the same result.

The important caveat: The LMP method assumes a standard 28-day cycle with ovulation on Day 14. If your cycle is consistently longer or shorter than 28 days, your actual ovulation – and therefore conception – occurred on a different day than the formula assumes. CycleWhisper’s due date calculator adjusts for your cycle length automatically, which makes the LMP-based result more accurate for women whose cycles do not follow the 28-day textbook average.

Method 2: Conception Date – The Most Precise Approach

If you know when you conceived – because you were tracking ovulation with OPK strips, a BBT chart, or CycleWhisper’s ovulation calculator – you can calculate your due date with greater precision than the LMP method allows.

The formula: Add 266 days (38 weeks) to your conception date.

Why 266 days instead of 280? Because conception-based dating starts from fertilization itself, not from your last period. The 14-day difference accounts for the two weeks between your LMP and ovulation that the LMP method includes.

A worked example:

• Estimated conception date: January 29th
• Add 266 days
• Estimated due date: October 22nd

This method is more precise because it removes the assumption about when ovulation occurred. If you have a 35-day cycle and used the LMP method without a cycle length adjustment, your calculated due date could be off by a full week or more. Using your confirmed ovulation or conception date sidesteps that variable entirely.

A practical note on conception date accuracy: Most women cannot identify the exact day of conception, even when tracking ovulation. Ovulation can occur slightly earlier or later than predicted, and sperm can survive for up to five days in the reproductive tract – meaning conception may have occurred up to five days after the intercourse that led to it. If your conception date is an estimate rather than a confirmed date, the LMP method with a cycle length adjustment is equally valid.

Method 3: IVF Transfer Date – The Most Precise of All

For women who conceived through in vitro fertilization (IVF), due date calculation follows a slightly different formula based on the embryo transfer date and the age of the embryo at the time of transfer.

IVF pregnancies are unique in that the exact day of fertilization is known, which makes due date calculation more precise than in naturally conceived pregnancies. The formula depends on whether you had a Day 3 embryo transfer or a Day 5 blastocyst transfer – the most common type in modern IVF cycles.

Day 5 blastocyst transfer: Add 261 days to the transfer date.

Day 3 embryo transfer: Add 263 days to the transfer date.

Day 6 blastocyst transfer: Add 260 days to the transfer date.

A worked example (Day 5 blastocyst):

• Transfer date: February 3rd
• Add 261 days
• Estimated due date: October 22nd

The logic behind these numbers: a Day 5 blastocyst is equivalent to five days past fertilization. Fertilization is equivalent to ovulation day in a natural cycle, which falls approximately 14 days after the LMP. So: 280 days (from LMP) − 14 days (to reach ovulation) − 5 days (embryo age) = 261 days from transfer.

IVF due dates calculated this way are typically very reliable because every variable – egg retrieval, fertilization date, and transfer date – is documented precisely. CycleWhisper’s pregnancy due date calculator includes a dedicated IVF tab for Day 3, Day 5, and Day 6 transfers.

Your Due Date and Your Trimesters

Once you have your due date, you can map out your entire pregnancy timeline. Pregnancy is divided into three trimesters, each representing a distinct phase of development.

Trimester	Weeks	What’s happening
First	Weeks 1–12	All major organs form; miscarriage risk highest; morning sickness typically peaks
Second	Weeks 13–26	Growth and movement; energy returns; anatomy scan around week 20
Third	Weeks 27–40	Final growth; lung maturation; preparation for birth

Key dates to calculate from your due date:

• End of first trimester: Due date minus 196 days (28 weeks before your due date)
• End of second trimester: Due date minus 98 days (14 weeks before your due date)
• Early term: 37 weeks (due date minus 21 days)
• Full term: 39–40 weeks (due date minus 7 days to due date)

You do not need to do this math manually. CycleWhisper’s pregnancy week-by-week tracker calculates all of these milestones from your due date automatically, and gives you a week-by-week breakdown of your baby’s development, size comparisons, symptom guides, and a to-do checklist for each stage of pregnancy.

What to Do Once You Have Your Due Date

A calculated due date is a starting point, not a final answer. Here is what to do with it immediately.

Book your first prenatal appointment. Most OB-GYNs and midwives recommend scheduling this between weeks 8 and 10. At this appointment your provider will confirm your due date, run initial bloodwork, and discuss your prenatal care plan. The earlier you get in, the better – popular providers can book out several weeks.

Schedule your dating scan. The first ultrasound – called a dating or viability scan – is typically performed between weeks 8 and 12. It confirms the pregnancy is progressing, checks for a heartbeat, and provides a due date based on fetal measurements. If your calculated due date and ultrasound date differ by more than seven days, your provider will adjust your official EDD accordingly.

Start prenatal vitamins if you have not already. Folic acid – 400 micrograms daily – is recommended from before conception through at least the first 12 weeks to reduce the risk of neural tube defects. This is one of the most evidence-backed interventions in prenatal care, per the CDC and ACOG.

Note your genetic screening window. First-trimester genetic screening – including the nuchal translucency ultrasound and cell-free DNA (cfDNA) testing – must be completed between specific gestational weeks. Missing the window means waiting for second-trimester options, which are less comprehensive.

How Your Due Date Might Change

It is entirely normal for a due date to be adjusted after an early ultrasound. This happens for two main reasons.

First, ultrasound-based dating using fetal measurements (crown-rump length, or CRL) is more accurate than calendar calculations in early pregnancy, particularly if your cycle is irregular or longer than 28 days.

Second, your LMP date may not be perfectly reliable – many women are unsure of the exact date, or experienced unusual spotting that they counted as a period when it was actually implantation bleeding.

If your due date shifts by a few days after your dating scan, that is completely normal and does not indicate anything is wrong with the pregnancy. It simply means your fetal measurements are refining the estimate.

Frequently Asked Questions

How many weeks pregnant am I if I know my due date?

Take today’s date and count backward from your due date. Your due date is 40 weeks from your LMP, so if your due date is October 22nd and today is April 22nd, you are approximately 20 weeks pregnant — exactly halfway. CycleWhisper’s pregnancy week tracker calculates your current gestational week automatically when you enter your due date.

What if I do not know the date of my last period?

Use your conception date (if you were tracking ovulation) or your IVF transfer date. If you have neither, an early ultrasound is the most reliable way to establish gestational age – your provider can measure the embryo and calculate backward from there. Do not guess at your LMP date, as an incorrect starting point produces an incorrect due date.

Can my due date change after my first ultrasound?

Yes, and it commonly does. If your ultrasound-based dating differs from your calculated due date by more than seven days in the first trimester (or more than 10 days in the second trimester), your provider will adjust your official EDD. This is standard practice and is a sign that your care team is being precise – not that something is wrong.

Is a due date the same as a birth date?

No. A due date is an estimate of the midpoint of a normal delivery window (37–42 weeks). Only about 4% of babies arrive on their exact due date. Most are born within one to two weeks of the EDD in either direction. Babies born between 39 and 40 weeks are considered full term; between 37 and 38 weeks is early term; and 41 weeks or beyond is late term.

Do twins have the same due date calculation?

Twin pregnancies use the same LMP or conception-based calculation for the initial due date, but the expected delivery window is typically earlier than a singleton pregnancy. The American College of Obstetricians and Gynecologists recommends delivery of uncomplicated dichorionic-diamniotic twins at 38 weeks rather than 40, and other twin types even earlier. Your provider will establish a twin-specific timeline at your first appointment.

Your due date is the anchor for your entire prenatal journey – the date everything else is scheduled around, measured against, and counted down to. Getting it right from the start means better-timed care, a clearer pregnancy timeline, and one less thing to stress about in those first overwhelming days after a positive test.

CycleWhisper’s pregnancy due date calculator handles all three methods – LMP, conception date, and IVF transfer – and gives you your due date, trimester milestones, and current gestational week in seconds. No sign-up, no subscription, no data stored.

Calculate my due date now →