How is EDD usually calculated?

Most due dates are estimated using Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP), assuming a 28‑day cycle. Our calculator also lets you adjust for different cycle lengths and use conception or IVF dates.

How accurate is an EDD calculator?

An EDD calculator gives an estimate, not a guarantee. Only about 4–5% of babies are born exactly on their due date. Most births occur between 37 and 42 weeks. Early ultrasound dating, especially in the first trimester, is usually more accurate than LMP alone.

Can I change my EDD after an ultrasound?

Sometimes. If an early ultrasound (usually before 13 weeks) differs significantly from the LMP‑based due date, your clinician may revise the official EDD based on the ultrasound measurements. Always follow the date your healthcare provider records in your chart.

What if my menstrual cycles are not 28 days?

If your cycles are longer or shorter than 28 days, ovulation and conception likely occurred earlier or later. Our calculator lets you enter your average cycle length and automatically adjusts the EDD by the difference from 28 days.

Is this EDD calculator suitable for IVF pregnancies?

Yes. Use the IVF/Embryo transfer tab and enter the transfer date and embryo age (day 3 or day 5). The calculator adds the appropriate number of days to estimate gestational age and due date. Your fertility clinic’s date should still be considered the official one.

Can I use this tool to plan induction or C‑section dates?

No. This calculator is for educational purposes only. Decisions about induction or C‑section timing must be made with your obstetrician or midwife, based on your medical history, baby’s growth, and professional guidelines.

EDD Calculator – Expected Date of Delivery from Last Period or Conception

What is EDD (Expected Date of Delivery)?

The expected date of delivery (EDD), often called the due date, is the estimated date when your baby is most likely to be born. It is usually calculated as 40 weeks (280 days) from the first day of your last menstrual period (LMP) in a typical 28‑day cycle.

Because ovulation, implantation, and fetal growth vary from person to person, the EDD is always an estimate. Only a small percentage of babies arrive exactly on that date, but it is still very useful for planning prenatal care and monitoring growth.

How this EDD calculator works

1. From last menstrual period (LMP)

The classic method uses Naegele’s rule. For a 28‑day cycle:

EDD = LMP + 280 days (40 weeks)

If your cycle length is different from 28 days, the calculator adjusts for earlier or later ovulation:

Cycle adjustment = (Your cycle length − 28) days
Adjusted EDD = LMP + 280 days + Cycle adjustment

2. From conception date

If you know the approximate conception date (for example from ovulation tracking or IUI), gestational age is defined as 2 weeks older than the embryo’s age. That means:

EDD = Conception date + 266 days (38 weeks)

3. From IVF / embryo transfer

For IVF pregnancies, we can start from the embryo transfer date and embryo age:

Day 3 embryo: conception equivalent ≈ transfer date − 1 day
Day 5 blastocyst: conception equivalent ≈ transfer date − 3 days

Once we have the conception equivalent date, we again add 266 days (38 weeks) to estimate the due date.

Understanding your pregnancy weeks and trimesters

The calculator also estimates your current gestational age and trimester based on today’s date:

First trimester: 0 weeks 0 days – 13 weeks 6 days
Second trimester: 14 weeks 0 days – 27 weeks 6 days
Third trimester: 28 weeks 0 days – birth

These boundaries are commonly used in obstetrics, but your provider may use slightly different cut‑offs.

Limitations and medical accuracy

Cycle‑based dating assumes regular ovulation, which may not be true if your cycles are irregular.
Early ultrasound (especially in the first trimester) is often more accurate than LMP alone.
Conditions such as IVF, multiple pregnancy, or certain medical issues may change how your EDD is determined.

Always treat this calculator as an educational guide. Your obstetrician or midwife is the best source for an official EDD and for decisions about induction, C‑section, or monitoring.

Frequently asked questions

Why is my ultrasound due date different from my LMP due date?

Ultrasound measures your baby’s size and compares it to large reference charts. If the ultrasound suggests a gestational age that differs significantly from the LMP‑based estimate, your clinician may adjust the official EDD to match the ultrasound, especially if the scan was done early in pregnancy.

Can my EDD change later in pregnancy?

Once an EDD is established (usually from early ultrasound or reliable LMP), it is rarely changed later. Later ultrasounds are less accurate for dating because babies grow at different rates.

What if I don’t know my LMP?

If you are unsure of your last period, your provider will usually rely on early ultrasound measurements and/or known conception or IVF dates to estimate gestational age and EDD.

Expected Date of Delivery (EDD) Calculator

EDD Calculator

Your pregnancy timeline (estimate)