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Medically Reviewed By Ferty9 Medical Board, at Ferty9 Fertility Center | Last Reviewed: Mar 25, 2026
Contents ↓ Introduction How it Works Results Accuracy Irregular Periods / PCOS 40-Week Roadmap Scan Other Tools References FAQs

Pregnancy Due Date Calculator

You just saw two pink lines – and your very next question is: “When will I meet my baby?”

That question deserves a real answer, not just a number. Your due date is the anchor for every prenatal appointment, every scan, and every milestone in the 40 weeks ahead. Use the calculator above to find your estimated due date and see how far along you are today – then read on to understand what your result means, why it might shift, and how to get an even more precise date if your cycle is irregular, you have PCOS, or you conceived through IVF.

How Your Due Date Is Calculated

There is no single formula that fits every pregnancy. The right method depends on what information you have. Here are the four approaches  –  from the most commonly used to the most precise.

Method 1: Last Menstrual Period (LMP)

This is the standard starting point for most pregnancies. We count 280 days (40 weeks) forward from the first day of your last period, using a formula developed by German physician Franz Naegele in the early 1800s.

The formula assumes a 28-day cycle with ovulation on Day 14. This works well for women with regular cycles. If your cycle is shorter or longer, the calculator adjusts your due date accordingly.

Example:

Last period started: 1 January 2026 Add 280 days → Estimated Due Date: 8 October 2026.
For a 30-day cycle, the calculator adds 2 extra days → EDD: 10 October 2026.

Method 2: Conception Date

If you know the date you conceived  –  because you were tracking ovulation, used an ovulation kit, or underwent fertility treatment  –  enter that date directly. We add 266 days (38 weeks) to give your EDD.

This method is more accurate than LMP for women whose cycle length varies month to month.

Method 3: IVF or Frozen Embryo Transfer (FET) Date

If you conceived through IVF, you have the most precise starting data possible. Fertilisation is a documented fact, not an estimate.

Transfer TypeHow We Calculate
Day-3 Embryo TransferAdd 263 days from the date of transfer
Day-5 Blastocyst TransferAdd 261 days from the date of transfer
Frozen Embryo Transfer (FET)Same formulas apply  –  Day 3 or Day 5 based on embryo age at freeze

The advantage of IVF dating is absolute certainty about fertilisation day. No need to estimate ovulation  –  your clinical file already contains the answer. At Ferty9, we treat this level of precision as the standard, not the exception.

Method 4: Early Ultrasound (The Gold Standard)

An ultrasound performed between 8 and 13 weeks  –  measuring Crown-Rump Length (CRL)  –  can pinpoint your baby’s gestational age to within 3 to 5 days. This is the most accurate method available, and it is particularly important for:

  • Women with irregular periods
  • Women with PCOS or PCOD
  • Anyone who does not remember their last period date
  • Pregnancies where LMP and early ultrasound dates disagree by more than 7 days
Why the first trimester scan is so accurate

Between 8 and 13 weeks, all babies grow at roughly the same rate regardless of genetics or maternal factors. This makes CRL measurement the most reliable ‘ruler’ available in early pregnancy. After 20 weeks, growth rates begin to vary between babies, making late scans less reliable for dating.

Understanding Your Result

What ‘How Far Along You Are’ Actually Means

After you enter your dates, the calculator shows two things: your estimated due date, and how many weeks and days pregnant you are today. That second number  –  your gestational age  –  is what doctors use to schedule every test, scan, and milestone in your pregnancy. Here is why it matters and what it is counting.

The Two-Week Gap  –  Why You’re ‘4 Weeks Pregnant’ When You Conceived 2 Weeks Ago

This confuses nearly every expectant parent. Here is the explanation:

Clinically, pregnancy is measured in Gestational Age, which starts counting from Day 1 of your last period  –  not from conception. At the moment of conception, you are already counted as 2 weeks pregnant by this standard.

This convention exists because, before pregnancy tests and ultrasounds, the last period was the only observable date doctors could use. The standard has remained consistent across international medicine ever since  –  which means all your prenatal screenings, scans, and milestones are scheduled against Gestational Age.

TermWhat It Means
Gestational AgeCounted from Day 1 of LMP. This is what your doctor uses. At 4 weeks gestational age, the embryo is 2 weeks old.
Fetal Age / Embryonic AgeThe actual biological age of the embryo  –  2 weeks less than gestational age.
EDD (Estimated Date of Delivery)The date 40 weeks from your LMP. Also shown on your scan reports.
EDC (Estimated Date of Confinement)An older clinical term for EDD. Same calculation, different name.

How Accurate Is Your Due Date?

Your EDD is a statistical midpoint, not a predicted event. Here is what the data actually shows:

Statistic Clinical Reality
Babies born on the exact EDD Only 4%  –  roughly 1 in 25
Babies born within 1 week of EDD Approximately 60%
Normal full-term delivery window 37 weeks to 42 weeks (5-week range)
LMP method accuracy Within 1–2 weeks for regular 28-day cycles
Early ultrasound accuracy (8–13 weeks) Within 3–5 days  –  the most precise dating method
For twin pregnancies Full term considered 37–38 weeks, not 40

This is why your due date is best thought of as the centre of a three-to-four week window, not a deadline. Your care team will monitor fetal wellbeing closely if your pregnancy extends past 40 weeks, using a Biophysical Profile (BPP) to assess the baby’s health.

Irregular Cycles, PCOS, and Due Date Accuracy

Standard due date calculators assume a 28-day cycle. If your cycle is irregular  –  or if you have PCOS or PCOD  –  the LMP method can misdate your pregnancy by anywhere from a few days to several weeks. This is not a minor difference: a wrong gestational age means wrongly timed screenings, which can lead to missed or unnecessary interventions.

The Clinical Risk of Inaccurate Dating with PCOS

Women with PCOS often ovulate later in their cycle than Day 14. A woman with a 40-day cycle who ovulates on Day 26 could be assessed as 2 weeks further along than she actually is if only LMP is used. At Ferty9, we always recommend a first-trimester dating scan for women with PCOS or irregular cycles to establish a reliable baseline before scheduling any downstream tests.

What to Do If Your Cycle Is Irregular

  • Use the LMP method as a starting estimate only
  • Book a dating scan between 8 and 10 weeks
  • Provide your average cycle length when using this calculator – even an approximate figure improves accuracy
  • If you have tracked ovulation and know when you conceived, use the Conception Date method instead

Your 40-Week Clinical Roadmap

Your EDD is not just a date – it is the reference point for every medical check across your pregnancy. Here is what your due date helps schedule:

Period Weeks Key Milestones Your EDD Determines
First Trimester Weeks 1–12 Viability scan (heartbeat, 6–8 weeks) | NT / Nuchal Translucency scan (11–13 weeks) | Dating confirmation | First prenatal blood panel
Second Trimester Weeks 13–26 Level II Anomaly Scan / TIFFA scan (18–20 weeks) | Quadruple screen blood test | Glucose tolerance test (~24–28 weeks) | Anatomy review
Third Trimester Weeks 27–40 Growth scans every 2–4 weeks | Group B Strep screening (35–37 weeks) | Biophysical Profile if past 40 weeks | Labour preparedness assessment
For Twin and Multiple Pregnancies

If you are expecting twins, your clinical timeline shifts. At Ferty9, we consider 37 to 38 weeks as full term for dichorionic twins, and 36 weeks for monochorionic twins. Growth monitoring begins earlier, and your anomaly scan may be scheduled at 16–18 weeks rather than 18–20. Use the calculator normally  –  but flag your multiple pregnancy to our team immediately so your care plan is adjusted.

Why Your Due Date Needs a Scan to Confirm

A calculator gives you an estimate. A dating scan gives you clinical certainty. These are not the same thing  –  and the difference matters when it comes to scheduling your NT scan, your anomaly scan, and your labour induction window.

At Ferty9, our precision dating approach treats the calculator as a first step  –  not the final word. For women with PCOS, irregular cycles, or an ART pregnancy, a first-trimester dating scan is not optional  –  it is the foundation of safe prenatal care, confirming the gestational age estimate your calculator gives you.

The Ferty9 Commitment

✓     Precision dating scans for PCOS, irregular cycles, and ART pregnancies
✓     IVF-specific due date calculation  –  Day-3 and Day-5 blastocyst protocols
✓     Over 15,000 families supported across 11 locations in Andhra Pradesh & Telangana
✓     Medical Board with decades of cumulative experience in fetal medicine and reproductive science
 
Schedule Your Precision Dating Scan: Connect with our experts at any Ferty9 location across Andhra Pradesh & Telangana.

Our Fertility Experts Says: Your pregnancy journey is unique and beautiful. While our pregnancy due date calculator gives you that exciting first glimpse into your timeline, remember that your healthcare team is your ultimate guide. At Ferty9 Fertility Clinics, we’re just here to help you start planning and dreaming about meeting your little miracle!

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References

  1. ACOG: Methods for Estimating the Due Date
  2. NIH: Naegele’s Rule and Pregnancy Duration
  3. March of Dimes: Pregnancy Week by Week

Disclaimer: This Pregnancy Calculator is for informational and educational purposes only. It does not provide medical advice, diagnosis, or treatment. The results are estimates based on user-provided information. For health concerns or issues, consultation with a qualified healthcare professional is recommended.