About This Pregnancy Due Date Calculator
Why due dates matter — and why they are estimates
Your estimated due date (EDD) is one of the most meaningful numbers in a pregnancy. It anchors every prenatal appointment, screening test, and preparation plan for the next nine months. But only about 5% of babies arrive on their exact due date. The EDD is a statistical midpoint: the 40-week mark (280 days) from the first day of your last menstrual period, based on an average cycle length of 28 days.
If your cycles are longer or shorter, your EDD shifts accordingly — this calculator adjusts automatically using the cycle length you enter. A 32-day cycle, for example, shifts the EDD four days later than the standard 28-day calculation. IVF patients get an even more precise calculation because the embryo's exact developmental age at transfer is known: a Day 3 embryo is 3 days old, so 263 days are added to the transfer date; a Day 5 blastocyst is 5 days old, so 261 days are added.
Whether your EDD shifts slightly after an early ultrasound or stays consistent throughout, it serves as your calendar anchor — for scheduling the nuchal translucency scan (weeks 11–14), the anatomy scan (weeks 18–22), the glucose screening (weeks 24–28), and the Group B strep test (weeks 35–37). Each of these has a specific window that matters clinically.
What week-by-week tracking actually means
Gestational age is counted from the first day of your last menstrual period — not from conception. That means in week 1 and week 2, you are not yet technically pregnant. Conception typically happens around day 14 of a 28-day cycle — the end of week 2. By week 4, the newly implanted embryo is producing hCG and most home pregnancy tests can detect it. By week 8, all major organ systems have begun forming and the embryo is officially a fetus.
Week 12 marks the end of the period of highest miscarriage risk, and many people choose this point to share their news. Week 20 is the halfway point and typically when the detailed anatomy scan is performed. Week 24 is the viability milestone — babies born this early have a chance of survival with intensive NICU care. Week 37 is "early full-term" — medically acceptable for delivery. Only 5% of babies are born exactly on their due date; around 80% are born within two weeks either side.
The fruit and vegetable size comparisons in this tool are a popular visual aid based on WHO fetal growth standards. Seeing that your baby went from a blueberry at week 7, to a lime at week 12, to a banana at week 20, to a cantaloupe at week 34 gives a concrete sense of the extraordinary pace of fetal growth. Actual measurements vary between pregnancies — these are midpoint estimates for a typical singleton pregnancy.
Frequently Asked Questions
What is the difference between gestational age and fetal age?
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Gestational age (what this calculator and all clinicians use) is counted from the first day of your last period — about 2 weeks before conception. Fetal age (sometimes called conceptional age) is counted from conception and is always about 2 weeks less. Because most people know their last period date rather than their exact conception date, gestational age is the universal clinical standard. All scan dates, screening windows, and milestone charts reference gestational age.
Can my due date change after an ultrasound?
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Yes. If your first trimester ultrasound (ideally at 8–12 weeks) shows a fetal crown-rump length that corresponds to a gestational age differing by more than 7 days from your LMP-based EDD, your provider may revise the due date. Early ultrasound is the most accurate dating method because fetal size variation is small at that stage. After 20 weeks, ultrasound dating becomes less accurate and the LMP-based or early ultrasound date is usually kept.
What does IVF Day 5 transfer mean for due date calculation?
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In IVF, embryos grow in a laboratory for 3 or 5 days before being transferred to the uterus. A Day 5 embryo (called a blastocyst) is already 5 days old from fertilization. So instead of adding the full 266 days from fertilization, the calculator adds 261 days (266 minus 5) from the transfer date. A Day 3 embryo is 3 days old, so 263 days are added. This makes IVF due dates significantly more precise than LMP-based estimates because the fertilization date is known exactly.
Is this tool a substitute for my doctor's advice?
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No. This tool provides educational estimates based on standard obstetric formulas and is not medical advice. Your OB, midwife, or fertility clinic will establish and update your official EDD based on ultrasound measurements and your full medical history. Always follow your provider's guidance on your pregnancy timeline, screening schedule, and any concerns that arise.
What cycle length should I use if my periods are irregular?
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Use your best estimate of your average cycle length over the past 3–6 months. If your cycles vary widely — for example between 24 and 38 days — an early ultrasound will give a more reliable due date than any LMP calculation. In that case, your provider will likely date the pregnancy from the crown-rump length measurement rather than the last period date.
How accurate are the fruit size comparisons?
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Fruit comparisons are popular visual aids based on approximate fetal measurements from WHO fetal growth standards. Length measurements switch from crown-rump (head to bottom) in weeks 1–19 to crown-heel (head to toe) from week 20 onward, which is why the numbers jump significantly at week 14. Actual fetal size varies between individuals and pregnancies. These comparisons are for educational illustration only and should not be used to assess whether your baby's growth is normal — your prenatal ultrasounds serve that purpose.