Pregnancy Due Date Calculator
Turn one key pregnancy date into a full timeline
Due dates, weeks pregnant and scan timing FAQ
How does this pregnancy due date calculator work?
The calculator takes either the first day of your last menstrual period (LMP) or an approximate conception or ovulation date and counts forward. For LMP it uses the classic 40-week (280-day) method. For conception it counts about 38 weeks (266 days). These are the same basic ideas behind most due date calculators and pregnancy wheels.
Is this due date the same as the one my midwife or doctor will use?
Not always. Your maternity team usually combines the date you give them with an early ultrasound scan. If the scan shows that baby is measuring differently from the calendar dates, the ultrasound-based date often becomes the official due date. This page is an educational estimate, not a medical decision tool.
Why is pregnancy counted from the last period instead of from conception?
Historically, it has been easier to remember the first day of a period than the exact moment of conception. Many guidelines define pregnancy length as 40 weeks from LMP, which assumes ovulation and conception happen roughly two weeks later in a 28-day cycle. When you enter conception, the calculator adds that “missing” two weeks back in so your week count matches typical clinical charts.
What if my menstrual cycles are not exactly 28 days?
In real life, many people have shorter, longer or irregular cycles. That means your real ovulation date might not line up perfectly with the simple 28-day model. The more irregular your cycles, the more important ultrasound and clinician review become. You can still use this page to get a ballpark and talk through timing with your midwife or doctor.
How accurate are due dates anyway?
Even with perfect information, only a small percentage of babies arrive exactly on their due date. Many healthy births happen in a window of a few weeks either side. The timeline here is best thought of as a guide to trimesters, scan windows and milestones rather than a promise about an exact day.
Does this calculator work for IVF, twins or medical conditions?
IVF pregnancies, twins or higher multiples, and pregnancies with medical complications are often dated and managed using more specific rules. This page does not handle embryo transfer dates or specialist protocols. Use it only as broad education and lean on your fertility or obstetric team for the real numbers and thresholds that apply to you.
Can I use this to decide when to induce labour or skip tests?
No. Induction timing, extra scans, blood tests and monitoring plans are medical decisions that depend on your health, baby’s growth and many other factors. This calculator is here to help you understand language like “20-week scan” or “third trimester,” not to replace medical advice or emergency care.
How to use this pregnancy due date calculator
The goal of this page is to turn one remembered date into a clear, sharable pregnancy timeline. Instead of holding scattered weeks and months in your head, you can see roughly where your due date lands, how far along you might be today, and when common scan windows sit on your calendar.
1. Pick the dating method that fits what you know
If you know the first day of your last menstrual period fairly confidently, choose LMP. This is the most common starting point in early pregnancy. If you closely tracked ovulation, used home insemination or know an approximate conception date, choose the conception/ovulation option instead. Both feed into the same 40-week-style counting, just from slightly different points.
2. Add your date and run the calculator
Use the date picker to enter your chosen day, then hit Calculate due date. The tool estimates your estimated due date (EDD), and, when the dates make sense, a rough “weeks + days pregnant today” number. It also highlights a typical term window so you can see the span during which many babies arrive.
3. Skim the week markers and scan windows
Under the headline summary you’ll see key points like end of the first trimester, typical 18–22 week anatomy scan slot, third-trimester threshold and your 40-week mark. The dates are approximate, but they can help you plan questions about screening, travel or work and discuss them calmly with your care team.
4. Copy the summary into your own planning tools
Use the Copy summary button to paste your snapshot into a notes app, pregnancy journal or a message to your partner or support person. The summary includes your input date, the method used, the estimated due date, rough gestational age and key windows, so you can refer back to it without re-entering everything.
5. Treat the dates as guide rails, not rules
Even with careful counting, every pregnancy is different. Ultrasound timing, your medical history, and baby’s growth pattern may all shift what your midwife or doctor considers the best working due date. If what you see here conflicts with what your team has given you, the medical version wins. Use this page to understand the conversation, not to argue with safety plans or urgent advice.
If you notice pain, bleeding, fluid loss, reduced movements later in pregnancy or any other worrying symptoms, skip the calculator and seek immediate medical care. Tools like this are for general education, not for diagnosing problems or deciding whether to get checked out.
How the pregnancy due date math works
Behind the scenes, this calculator uses familiar obstetric shortcuts so you can see how clinicians think about gestational age and due dates without needing a pregnancy wheel. The numbers are simple on purpose, and they are always a starting point, not a final verdict.
1. Counting 40 weeks from the last menstrual period
When you choose LMP, the tool adds 280 days (40 weeks) to the first day of your last period. This matches Naegele-style rules used in many clinics and public calculators. It assumes a 28-day cycle where ovulation happens around day 14, so conception is roughly two weeks after the LMP date you enter.
2. Counting 38 weeks from conception
When you choose conception, the calculator adds about 266 days (38 weeks). It then “adds back” the two weeks that clinical dating usually counts before ovulation so that your week number lines up with standard charts. That is why your gestational age can be a couple of weeks longer than the time since you think you conceived.
3. Working out weeks and days pregnant
For both methods the tool builds an internal LMP-style start date and compares it with today’s date. The difference in days is broken into whole weeks plus extra days. If today falls outside a typical 0–42 week range, the calculator flags that instead of pretending to know your exact gestational age.
4. Building a simple week-by-week timeline
Once it has that internal start date, the calculator steps through key weeks and attaches approximate calendar dates to them—points like week 12, week 20, week 28, week 32, week 36 and week 40. The short notes beside each reflect common teaching on when first-trimester screens, anatomy scans or third-trimester checks often happen, but your own schedule may look different.
Because cycle length, ultrasound findings and medical needs vary, treat everything you see here as a rounded estimate. The safest and most accurate version of your due date and pregnancy plan is the one you work out together with your midwife, obstetrician or local maternity service.
References and further reading on pregnancy due dates
These resources explain common methods for estimating due dates and why ultrasound and clinical judgment matter:
- Mayo Clinic — Pregnancy due date calculator: How is your due date calculated? — outlines how LMP-based counting and early ultrasound are used together to estimate gestational age.
- NHS — Due date calculator — describes using the first day of the last period to estimate due date and highlights the typical 37–42 week birth window.
- American College of Obstetricians and Gynecologists — Methods for estimating the due date — explains how Naegele-style calculations, ultrasound findings and clinical context come together to determine an official EDD.
Use these as background reading, then rely on your own maternity team for specific dating, screening and birth planning decisions.