About the Pregnancy Due Date Calculator
Tezmy's Pregnancy Due Date Calculator estimates when your baby is due and how far along you are. Enter the first day of your last period (with your usual cycle length), the date of conception, an IVF embryo transfer date or an ultrasound result, and it uses the standard rules doctors use: about 280 days (40 weeks) from the start of the last period. You see your estimated due date, how many weeks and days pregnant you are today, which trimester you're in, how many days are left, and a timeline of key weeks such as the usual scan windows. It is an estimate to help you plan; your doctor or midwife will confirm your dates. Nothing you enter leaves your device.
Updated September 2026
How to use the Pregnancy Due Date Calculator
- 1Choose how to calculate: last period, conception, IVF or ultrasound.
- 2Enter the date (and your cycle length, embryo age or scan result if asked).
- 3Read your estimated due date, weeks pregnant and trimester.
- 4Scroll down for the week-by-week timeline of key dates.
Which method to use
- Last period: the most common way, if you remember the first day of your last period and have fairly regular cycles.
- Conception: if you know when you conceived, for example from ovulation tracking.
- IVF: the transfer date and embryo age give a very precise date.
- Ultrasound: use the date of your scan and the age it showed; early scans are the most accurate.
Understanding the timeline
The timeline lists weeks that often matter during pregnancy, such as the usual windows for the dating scan and the anatomy scan and when each trimester starts. Scan timings differ between countries and clinics, so follow the schedule your doctor or midwife gives you. The dates here simply help you see when each week falls on the calendar.
A note on care
This calculator is for information and planning, not medical advice. Book an appointment with a doctor or midwife as soon as you know you are pregnant, take any supplements they recommend, and contact them straight away if you have bleeding, severe pain or anything that worries you.
Handy for planning
- Count the days until the due date with Days Between Dates.
- Keep track of appointments and questions for your doctor in the Online Notepad.
- Check healthy weight ranges with the BMI Calculator before pregnancy, and ask your doctor about weight gain during it.
Features
- Four methods: last period, conception, IVF, ultrasound
- Adjusts for longer or shorter cycles
- Day-3 and day-5 IVF transfers
- Weeks and days pregnant today
- Trimester and days to go
- Timeline of key weeks and usual scan windows
- Private: worked out on your device
Frequently asked questions
How is the due date calculated?
By convention, pregnancy is counted from the first day of your last menstrual period, and the due date is 280 days (40 weeks) later. If your cycle is longer or shorter than 28 days, the date moves by the difference. From a known conception date it is 266 days, and after IVF it is 261 days from a day-5 transfer or 263 days from a day-3 transfer.
How accurate is a due date?
It is an estimate. Only a small share of babies are born on the exact date; most arrive in the two weeks before or after it. An early ultrasound (before 14 weeks) is the most accurate way to date a pregnancy, and doctors may change your due date based on it.
How many weeks pregnant am I?
Enter your dates and the calculator shows weeks and days pregnant for today. The count starts from the first day of your last period, about two weeks before conception, so at conception you are already “2 weeks pregnant” by this standard.
What if I don't know the date of my last period?
Use the date of an ultrasound scan and the pregnancy age it showed (for example 12 weeks 3 days), or ask your doctor. If you know when you conceived, use the conception method.
When do the trimesters start?
The first trimester lasts until the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. The timeline shows the dates for your pregnancy.
Is my information stored?
No. The dates are worked out in your browser and are not sent anywhere or saved. Close the page and they are gone.