Published: February 2026 | Last Updated: February 2026
Calculate your estimated due date using the standard 280-day method. Understand Naegele's rule, trimester milestones, and what to expect throughout your pregnancy journey.
Your estimated due date (EDD) is typically calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This method, known as Naegele's rule, has been the standard for pregnancy dating for over 200 years and remains widely used by healthcare providers today.
It's important to remember that a due date is an estimate—only about 5% of babies are born on their exact due date. Most healthy pregnancies result in birth anywhere from 38 to 42 weeks.
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. Since ovulation typically occurs about 14 days after LMP, and conception happens around ovulation, a "40-week pregnancy" actually represents about 38 weeks of fetal development.
This dating convention exists because:
Named after German obstetrician Franz Naegele (1778-1851), this rule provides a quick way to estimate due date:
This is mathematically equivalent to adding 280 days to LMP.
Pregnancy is divided into three trimesters, each with distinct developments and experiences:
Fertilisation and implantation occur. Major organs begin forming. By week 12, the baby is about 6cm long. Common symptoms include morning sickness, fatigue, and breast tenderness.
Often called the "golden period"—energy returns and morning sickness usually subsides. You may feel baby's first movements (quickening). Gender can be determined via ultrasound.
Baby gains weight rapidly and prepares for birth. You may experience Braxton Hicks contractions, increased tiredness, and frequent urination. Baby typically moves into head-down position.
Our date calculator can help with all pregnancy-related calculations—add 280 days, track trimester milestones, and more.
Open Date Calculator| Week | Days from LMP | Key Development |
|---|---|---|
| Week 4 | 28 days | Implantation complete; pregnancy test may show positive |
| Week 6 | 42 days | Heart begins beating; early ultrasound may detect heartbeat |
| Week 8 | 56 days | All major organs forming; embryo becomes a fetus |
| Week 12 | 84 days | End of first trimester; risk of miscarriage decreases |
| Week 16 | 112 days | Baby can hear sounds; you may start to show |
| Week 20 | 140 days | Anatomy scan; halfway point; may feel movements |
| Week 24 | 168 days | Viability milestone; baby has chance of survival if born |
| Week 28 | 196 days | Third trimester begins; baby opens eyes |
| Week 32 | 224 days | Baby practices breathing; rapid brain development |
| Week 36 | 252 days | Baby is almost full term; may drop into pelvis |
| Week 37 | 259 days | Early term; baby is considered ready for birth |
| Week 40 | 280 days | Due date; full term pregnancy complete |
Our free date calculator is perfect for pregnancy-related calculations:
If you know your due date and want to estimate conception:
Several factors can affect how accurate your calculated due date is:
Naegele's rule assumes a 28-day menstrual cycle with ovulation on day 14. If your cycles are longer or shorter, your due date may differ:
First-trimester ultrasounds (before 13 weeks) are the most accurate way to date a pregnancy. They measure the crown-rump length of the embryo/fetus and can estimate due date within 5-7 days. If ultrasound dating differs significantly from LMP dating, your healthcare provider may adjust your due date.
For IVF pregnancies, the date of embryo transfer is known precisely, allowing for more accurate due date calculation. Your fertility specialist will provide specific dating based on your treatment timeline.
This calculator provides an estimate for informational purposes only. Always consult with your healthcare provider for accurate pregnancy dating and medical advice.
While your due date gives you a target, the reality of birth timing varies:
| Classification | Weeks | Description |
|---|---|---|
| Preterm | Before 37 weeks | Baby may need special care |
| Early Term | 37-38 weeks | Baby is mature but benefits from more time |
| Full Term | 39-40 weeks | Ideal time for birth |
| Late Term | 41 weeks | Close monitoring recommended |
| Post Term | 42+ weeks | Induction often recommended |
How do I calculate my pregnancy due date?
The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Use our calculator above or the date calculator to find your estimated due date.
Why is pregnancy calculated from LMP, not conception?
LMP is used because it's usually known with certainty, while the exact date of conception is often unknown. This provides a consistent starting point. Conception typically occurs about 2 weeks after LMP.
How accurate is a due date calculator?
Due date calculators provide an estimate. Only about 5% of babies arrive on their exact due date—most are born within 2 weeks before or after. First-trimester ultrasounds provide the most accurate dating.
What if I don't know my LMP?
If you're unsure of your LMP, an early ultrasound (ideally before 13 weeks) can accurately date your pregnancy. Speak with your healthcare provider about scheduling a dating scan.
How many weeks is a full-term pregnancy?
Full-term pregnancy is considered 39-40 weeks. Babies born at 37-38 weeks are "early term," while those born after 41 weeks are "late term." Post-term is 42+ weeks.
What is the difference between gestational age and fetal age?
Gestational age is counted from LMP (40 weeks). Fetal age is counted from conception (about 38 weeks). Healthcare providers typically use gestational age.
Can my due date change?
Yes. If an early ultrasound shows a significant difference from LMP dating (usually more than 5-7 days), your healthcare provider may adjust your due date. This is most accurate when done in the first trimester.
How do I calculate when I conceived?
Conception typically occurs about 14 days after LMP (during ovulation), or about 266 days before your due date. However, the exact date can vary based on your cycle length.
Calculate any pregnancy milestone, track trimester dates, or plan around your due date with our easy-to-use tool.
Open Date Calculator