How to Calculate Your Due Date
Your estimated due date (EDD) is the date your baby is expected to arrive. Most pregnancies last about 280 days (40 weeks) from the first day of the last menstrual period (LMP). This is the method most doctors use, known as Naegele's Rule.
Due Date = First day of last period + 280 days
Or equivalently: LMP + 9 months + 7 days
Keep in mind that only about 5% of babies are born on their actual due date. Most arrive within a two-week window on either side.
Three Ways to Estimate Your Due Date
1. From Your Last Menstrual Period (LMP)
This is the most common method. Count 280 days (40 weeks) from the first day of your last period. This assumes a 28-day cycle with ovulation on day 14.
Example: If your last period started on January 1, your due date is October 8 (January 1 + 280 days).
2. From Conception Date
If you know when you conceived (or your ovulation date), add 266 days (38 weeks). This is more accurate for women with irregular cycles.
Due Date = Conception date + 266 days
3. From Ultrasound
A first-trimester ultrasound (before 13 weeks) can estimate the due date within about 5-7 days by measuring the baby's size. This is the most accurate method and may adjust the LMP-based estimate.
Pregnancy Timeline by Trimester
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First | 1-12 | Positive test (~4 weeks), heartbeat visible (~6 weeks), all major organs forming, nausea common |
| Second | 13-27 | Energy returns, baby movements felt (~18-22 weeks), anatomy scan (~20 weeks), gender can be determined |
| Third | 28-40 | Baby gains weight rapidly, lungs mature, nesting instinct, Braxton Hicks contractions, full term at 39 weeks |
Due Date by Month of Last Period
Quick reference — if your last period started in a given month, here's approximately when your baby is due:
| Last Period Month | Due Date Month |
|---|---|
| January | October |
| February | November |
| March | December |
| April | January (next year) |
| May | February |
| June | March |
| July | April |
| August | May |
| September | June |
| October | July |
| November | August |
| December | September |
Factors That Affect Your Due Date
- Cycle length: The 280-day rule assumes a 28-day cycle. If your cycle is 35 days, ovulation likely occurred around day 21 instead of day 14, pushing your true due date about a week later.
- First pregnancy: First-time mothers tend to deliver slightly later on average (about 1-3 days past the due date).
- Multiple pregnancies: Twins typically arrive around 36-37 weeks, and triplets around 33-34 weeks.
- Previous pregnancies: If your previous babies came early or late, this pregnancy may follow a similar pattern.
When Is "Full Term"?
The definition of full term has been refined in recent years:
| Classification | Weeks | Notes |
|---|---|---|
| Early term | 37-38 weeks | Baby is developed but brain and lungs benefit from more time |
| Full term | 39-40 weeks | Ideal time for delivery |
| Late term | 41 weeks | Still normal, doctor may discuss induction |
| Post-term | 42+ weeks | Increased monitoring, induction typically recommended |
Frequently Asked Questions
How accurate is a due date?
A due date is an estimate. Only about 5% of babies arrive on the exact due date. About 80% are born within 2 weeks before or after. First-trimester ultrasound dating is accurate to within 5-7 days.
Can my due date change?
Yes. If an early ultrasound shows the baby is measuring significantly different from the LMP-based estimate (more than 5-7 days difference), your doctor may adjust the due date to match the ultrasound measurement.
How do I calculate due date with irregular periods?
If your periods are irregular, the LMP method may not be accurate. A first-trimester ultrasound is the best way to establish your due date. Alternatively, if you tracked ovulation (using tests, temperature, or cervical mucus), count 266 days from your ovulation date.
Is it safe to deliver at 37 weeks?
While babies born at 37 weeks usually do well, research shows that waiting until 39 weeks (when possible) gives the baby's brain, lungs, and liver important final development time. The American College of Obstetricians recommends against elective delivery before 39 weeks unless medically indicated.