EDD Calculator Formula: How Your Date Is Worked Out
Pregnancy is counted from the first day of your last period, not from the day of conception. That is why, on the day you conceive, you are already counted as roughly two weeks pregnant.
- From your last period: the calculator adds 280 days (40 weeks) to the first day of your LMP to give the EDD.
- If your cycle is not 28 days: a longer cycle moves the date later by the extra days; a shorter cycle moves it earlier by the same amount.
- From a known conception date: 266 days are added to that date.
- After IVF: the date is worked out from the day of embryo transfer (see the IVF section below).
EDD by USG: Why I Rely on the Early Scan
A baby's growth in the first trimester is remarkably uniform, so an ultrasound at this stage gives the most accurate due date. Measuring the crown-rump length (CRL) between 7 and 13+6 weeks pins the pregnancy down to within about five days. For this reason I advise a dating scan and an NT scan even when everything is going smoothly.
Due Date Calculator from Conception Date: When You Know the Day
You may know the day you conceived, for example because you were tracking ovulation. Then choose "date of conception" in the calculator, and it adds 266 days (38 weeks). The two methods agree for a regular 28-day cycle, because ovulation then usually happens around day 14, which is why the LMP rule adds two extra weeks. Keep in mind that conception does not always happen on the day of intercourse. Sperm can survive for a few days, and the egg is usually fertilised within about a day of ovulation. A date of conception is therefore often an estimate as well. When the conception date, the period date and the early scan disagree, I usually go by the scan.
Due Date Calculator for IVF Pregnancies: Counting from Embryo Transfer
In IVF the embryo's age on the day of transfer is known exactly, so the due date is more precise than in a natural conception. The rule starts from the transfer date and allows for the embryo's age on that day. A day-3 embryo transfer adds 263 days, and a day-5 blastocyst transfer adds 261 days. The same rule applies to a frozen embryo transfer, counting from the transfer date and using the embryo's age when it was frozen. This calculator has no separate IVF setting. To use it after a fresh transfer, enter the day of egg collection as your date of conception. After a frozen transfer, enter the date three days (day-3 embryo) or five days (day-5 blastocyst) before the transfer date. Both give the same result as the rule above. Your fertility team's date should still be the one you follow.
Your Pregnancy, Trimester by Trimester
| Trimester | Weeks | What usually happens in this period |
|---|---|---|
| First | 0–13 | Dating scan; NT scan and first-trimester screening; folic acid; checks of blood pressure, thyroid and haemoglobin |
| Second | 14–27 | Level II anomaly scan at 18–22 weeks; fetal echo if it is advised; glucose test for gestational diabetes |
| Third | 28–40 | Growth and Doppler scans; keeping track of your baby's movements; planning the delivery |
Call Straight Away - Whatever Your Dates Say
Do not wait for your next scheduled visit if you notice:
- bleeding, or severe pain in the abdomen
- a headache that will not settle, blurred or flashing vision, or swelling of the face and hands that comes on suddenly
- fluid leaking from the vagina, or your baby moving less than usual
Call +91 90545 74829. Emergency and labour care at Divine Women's Hospital is available 24 × 7.
Due Date Questions Patients Often Ask Me
How does the calculator work out my due date?
It uses Naegele's rule. 280 days are added to the first day of your last period, assuming a 28-day cycle. The date moves later or earlier if your cycle is longer or shorter. If you know the date of conception, 266 days are added to that date instead.
How reliable is an online due date calculator?
It gives an estimate, not a fixed date. Only about 4–5% of babies are born on their due date; most arrive within two weeks either side of it. An ultrasound between 7 and 13+6 weeks is the most accurate way to date a pregnancy.
My scan gives a different due date from the calculator. Which should I follow?
If the early scan differs from the period-based date by more than about 5–7 days, the scan date is generally used, because periods are often irregular or remembered inexactly. Scans later in pregnancy usually do not change the due date.
How early should I book my first pregnancy appointment?
Ideally at 6–8 weeks. If you have a long-term illness, had problems in an earlier pregnancy, or have pain or bleeding, come as soon as the test turns positive. Keep in mind that the NT scan has to be done between 11 weeks and 13 weeks 6 days.
Is the due date calculated differently in an IVF pregnancy?
Yes. It is based on the date of embryo transfer: add 266 days, minus the age of the embryo at transfer - for a day-5 blastocyst, that means adding 261 days. Your fertility team will give you the exact date.