How is my pregnancy due date calculated?
Pregnancy dating is commonly estimated by counting 40 weeks (280 days) from the first day of your last menstrual period — a method known as Naegele's rule. It assumes an average 28-day cycle with ovulation around day 14. If you know your ovulation or conception date instead, the calculator can use that directly, since pregnancy is typically estimated at 38 weeks (266 days) from conception. Cycle length can shift the estimate, which is why the last-period method lets you adjust it.
How to calculate your due date from your last period
The first day of your last menstrual period is the most commonly used starting point for pregnancy dating, mainly because it's the one date most people can pinpoint with confidence. A typical estimate is around 40 weeks from that date. Since not everyone has a 28-day cycle, this calculator lets you adjust the cycle length — a longer or shorter cycle shifts the estimated due date accordingly.
How to calculate your due date from ovulation
If you know roughly when you ovulated — for example from tracking, an ovulation test, or a fertility app — you can use that date directly as an alternative to your last period. Pregnancy is generally estimated at 38 weeks from conception, which usually happens within a day of ovulation. This can be a more precise starting point if your cycles are irregular and last-period dating is less reliable, though ovulation timing itself is still an estimate for most people rather than an exact figure.
How to calculate your due date from conception
If you have a known or approximate conception date — for example from fertility treatment or tracking — you can enter it directly. The calculator estimates your due date as 38 weeks from that date. As with ovulation dating, treat the conception date as your best estimate rather than an exact, guaranteed figure unless it comes from a clinical procedure with a known date.
How many weeks pregnant am I?
Pregnancy weeks are generally counted from the first day of your last menstrual period, not from the day you actually conceived. Because ovulation and conception usually happen around two weeks after your period starts, "weeks pregnant" is typically about two weeks ahead of the actual time since conception. So if you're told you're "6 weeks pregnant," conception likely happened around 4 weeks ago, not 6.
How accurate is a pregnancy due date calculator?
A due date calculator gives a solid estimate, not a guarantee. Several things affect its accuracy: menstrual cycle length varies from person to person and cycle to cycle, ovulation doesn't occur on exactly the same cycle day for everyone, and the standard 40-week estimate is an average, not a fixed rule. Only around 1 in 20 babies are born on their exact estimated due date — most arrive within a couple of weeks either side. A healthcare professional can refine your due date, and an early ultrasound dating scan is generally considered more precise than calculation from dates alone.
What if my menstrual cycle is not 28 days?
28 days is a common reference cycle length, but individual cycles vary quite a bit — anywhere from about 21 to 35 days is typical. A shorter or longer cycle usually means ovulation happens earlier or later relative to the start of your period, which shifts your estimated due date. That's why the last-period mode above lets you set your own average cycle length rather than assuming 28 days for everyone.
Can my due date change?
Yes. An initial estimated due date is often adjusted after a clinical assessment — most commonly an early ultrasound, which is generally considered more accurate than dating from your last period alone, especially if your cycles are irregular. It's normal for the estimate to shift slightly as your pregnancy progresses and more information becomes available. If a healthcare professional gives you a different due date than this calculator, their assessment should take priority.
Important information
This calculator provides an estimate for general informational purposes only. It does not diagnose pregnancy and does not provide medical advice. Please discuss pregnancy dating, and any concerns about your pregnancy, with your doctor or midwife. If the date calculated here differs from a date given to you by a healthcare professional, follow their guidance rather than this tool.
Frequently asked questions
How do I calculate my due date?
Choose whichever date you know: your last menstrual period, your ovulation date, or your conception date. Enter it above and the calculator estimates your due date, how many weeks pregnant you are, your trimester, and your estimated conception date.
Can I calculate my due date from ovulation?
Yes — switch to the "Ovulation Date" tab above and enter the date you ovulated. The calculator estimates your due date as 38 weeks from that date.
Can I calculate my due date from my last period?
Yes, this is the default method. Enter the first day of your last period and your average cycle length, and the calculator applies the standard 40-week estimate, adjusted for your cycle.
What if I don't remember my last period?
If you know your approximate ovulation or conception date instead, use one of those tabs above. If you don't know any of these dates precisely, a healthcare professional can help estimate dating, often via an early ultrasound.
How many weeks pregnant am I?
Enter your last period, ovulation, or conception date above and the "How Far Along" result shows your current estimated weeks and days.
Can my due date change?
Yes — see the "Can my due date change?" section above. It's common for an estimate to be refined after a clinical dating scan.
Is a due date exactly 40 weeks?
40 weeks from your last period (or 38 weeks from conception) is the standard estimate, but it's a midpoint, not a deadline. A full-term birth falls anywhere from about 37 to 42 weeks, and only around 1 in 20 babies arrive exactly on their estimated due date.
What is the difference between conception date and due date?
Your conception date is when pregnancy actually began, roughly two weeks after the first day of your last period. Your due date is the estimated date of birth, calculated as about 40 weeks from your last period or 38 weeks from conception.
How accurate is a pregnancy due date calculator?
It's a useful estimate based on averages, not an exact prediction — see the accuracy section above for the main factors that affect it.
What if my menstrual cycle is longer or shorter than 28 days?
Use the cycle length slider in the last-period tab above to enter your actual average cycle length; the calculator adjusts your estimated due date accordingly.
Medical information sources
This page follows general dating conventions described by major health organisations. For authoritative detail beyond this estimate, see the NHS guide to working out your due date, the Mayo Clinic's pregnancy due date calculator guidance, and the American College of Obstetricians and Gynecologists' (ACOG) information on pregnancy dating. SolveSmart is not affiliated with or endorsed by these organisations; they're provided as independent references.