One of the first questions after finding out you are pregnant is: “When is my due date?” While no due date is exact — only about 5% of babies arrive on their exact due date — having an estimated date helps you plan prenatal appointments, track fetal development, and prepare for the big day.
Use our pregnancy due date calculator to get your estimated due date instantly. Just enter the first day of your last menstrual period, and the tool calculates the rest.
How Due Dates Are Calculated
The standard method for calculating a due date is called Naegele’s Rule. It works like this: take the first day of your last menstrual period (LMP), add 280 days (40 weeks), and you have your estimated due date. This 40-week count assumes a 28-day menstrual cycle with ovulation on day 14.
| Step | Calculation | Example |
|---|---|---|
| 1. Start with LMP | First day of last period | January 1, 2026 |
| 2. Add 280 days | Add 9 months + 7 days | October 8, 2026 |
| 3. Adjust for cycle | If cycle > 28 days, add the difference | +3 days for 31-day cycle |
Alternative Calculation Methods
Not all pregnancies follow the standard LMP method. Here are other ways healthcare providers estimate due dates:
- Ultrasound dating: First-trimester ultrasounds (8-13 weeks) measure the crown-rump length of the fetus. This is the most accurate method, within 5-7 days.
- Conception date: If you know the exact date of conception, add 266 days (38 weeks). This works well for IVF pregnancies.
- Fundal height measurement: After 20 weeks, your doctor measures the distance from the pubic bone to the top of the uterus, which roughly matches the week of pregnancy.
Pregnancy Tracker: What Happens Each Trimester
First Trimester (Weeks 1-13)
- Weeks 4-5: The embryo implants and the heart begins to beat.
- Weeks 8-10: All major organs have formed. The embryo is now called a fetus.
- Weeks 12-13: Morning sickness often peaks and starts to subside. The risk of miscarriage drops significantly after week 12.
Second Trimester (Weeks 14-27)
- Weeks 16-20: Fetal movement (quickening) is usually felt. Anatomy ultrasound checks all organs.
- Weeks 24-26: The fetus has a chance of survival outside the womb with medical support (viability).
- Week 27: The baby can open and close its eyes and respond to sound.
Third Trimester (Weeks 28-40)
- Weeks 32-36: The baby grows rapidly and settles into a head-down position.
- Weeks 37-40: Full term. The baby is considered ready for birth. Only 5% of babies arrive on their exact due date; most come between 38 and 42 weeks.
Factors That Affect Due Date Accuracy
- Irregular cycles: Women with cycles longer or shorter than 28 days may ovulate earlier or later, shifting the true due date.
- Multiple pregnancies: Twins and triplets are usually delivered earlier, around 36-37 weeks for twins.
- Maternal age: Women over 35 may have different due date considerations for monitoring purposes.
- First-time vs subsequent pregnancy: First-time pregnancies tend to go slightly longer on average.
Try our pregnancy due date calculator to get your estimate, and use it alongside your healthcare provider’s professional assessment for the most accurate planning.
