Pregnancy Due Date Calculator Online
Pregnancy Due Date Calculator
Due Date Calculator
The first day of your last period
Typical range: 21-35 days
The due date calculator estimates your baby's arrival date based on the first day of your last menstrual period (LMP). It uses Naegele's Rule, the same method used by doctors and midwives worldwide. Most pregnancies last about 40 weeks from the LMP date.
Naegele's Rule
Naegele's Rule adds 280 days (40 weeks) to the first day of your last period. It was developed in the 19th century by German obstetrician Franz Naegele and remains the standard clinical method today.
Due Date = LMP + 280 days
Alternate calculation: Take the first day of your LMP, subtract 3 months, add 7 days, and add 1 year. Both methods give the same result.
Pregnancy Trimester Breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First Trimester | Week 1 to Week 12 | Organ formation, heartbeat starts around week 6 |
| Second Trimester | Week 13 to Week 26 | Fetal movement, anatomy scan at week 18-20 |
| Third Trimester | Week 27 to Week 40 | Rapid growth, lung maturation, birth preparation |
Gestational Age vs. Fetal Age
Gestational age counts from the first day of your last period. Fetal age (conceptional age) counts from the actual day of conception, which is typically about 2 weeks into your cycle. Doctors and ultrasound reports always use gestational age. When a scan says your baby is 20 weeks, that means 20 weeks gestational, not 20 weeks from conception.
Due Date Accuracy
Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks before or after. An ultrasound in the first trimester (before 14 weeks) is more accurate than the LMP method, especially if your cycles are irregular. If the scan date differs from the LMP date by more than 5-7 days, your doctor may adjust your due date.
Frequently Asked Questions
How is a pregnancy due date calculated?⌄
Add 280 days (40 weeks) to the first day of your last menstrual period. This is called Naegele's Rule and is the standard method used globally. The due date is an estimate. Only about 5% of babies arrive on the exact predicted date.
What if my cycles are irregular?⌄
The LMP method assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, your due date may shift. An early ultrasound (before 14 weeks) gives a more accurate due date by measuring the embryo directly.
How is due date calculated after IVF?⌄
For IVF, the due date is calculated from the egg retrieval date plus 266 days, or from the transfer date plus 261 days for a 5-day blastocyst transfer. Your fertility clinic provides the exact retrieval or transfer date to use.
What does 40 weeks pregnant mean?⌄
40 weeks pregnant means you are at your estimated due date. The 40 weeks count starts from the first day of your last period, so the baby has actually been developing for about 38 weeks. Full-term pregnancy is considered 39 to 40 weeks and 6 days.
Can I find out my due date from a positive pregnancy test?⌄
No. A positive test only confirms pregnancy, not how far along you are. You need the date of your last menstrual period or an ultrasound to calculate gestational age and due date.