Pregnancy due date math — counting from LMP vs conception
Pregnancy due dates are calculated from the last menstrual period (LMP), not from conception. The standard rule: add 280 days (40 weeks) to the LMP. This gives a due date estimate. Refinements come from ultrasound dating.
For computing duration between two dates (useful for tracking pregnancy weeks), the duration calculator handles it.
The basic math
LMP: First day of last menstrual period (menstrual cycle).
Add 280 days (40 weeks): this is the standard estimated due date (EDD).
Why 280 days? Because:
- Average pregnancy length is 40 weeks
- Counting starts from LMP, which is 2 weeks before conception
- "40 weeks pregnant" actually means 38 weeks of fetal growth
- The convention has been used in obstetrics for decades
For a woman with LMP March 1, 2026:
- 40 weeks = 280 days
- Due date estimate: December 6, 2026
Why count from LMP, not conception?
Several reasons:
1. LMP is easier to identify. Most women track menstrual periods. Conception is harder to pinpoint precisely.
2. The convention was set before precise conception dating was possible. Modern technology can identify conception within hours, but the LMP convention persists.
3. Statistical average. The "40 weeks from LMP" estimate is reasonable for typical menstrual cycles. Variations exist but the average is close.
4. Standard medical recording. Hospitals and electronic health records use LMP as the standard reference. Switching would require re-coding decades of medical records.
Conception is roughly 14 days after LMP
In a typical 28-day menstrual cycle, ovulation happens around day 14 (counting from LMP). Conception happens within 24 hours of ovulation.
So if LMP is March 1:
- Day 14 (March 15) = ovulation, possible conception
- Day 21 (March 22) = embryo implants
- Day 28 (March 29) = expected next period; if it doesn't come, suspect pregnancy
- Day 280 (December 6) = estimated due date
The "fetal age" is about 38 weeks (counting from conception); the "gestational age" is about 40 weeks (counting from LMP). Medical records use gestational age.
Variations in cycle length
The 40-week rule assumes a 28-day menstrual cycle. For women with longer or shorter cycles:
Long cycle (35+ days): ovulation is delayed; conception happens later. Due date might be 2-3 days later than the simple LMP+280 calculation.
Short cycle (less than 28 days): ovulation earlier; conception earlier. Due date might be 2-3 days earlier.
Irregular cycles: standard calculation may be off by a week or more. Ultrasound dating becomes important.
For typical 28-30 day cycles, the LMP+280 rule is accurate enough.
Ultrasound dating
In modern medical practice, ultrasound dating is used to refine the due date:
1st trimester ultrasound (around weeks 8-12): very accurate (within ±5-7 days). Measurements of fetal length correlate strongly with gestational age.
2nd trimester ultrasound (weeks 18-22): less accurate (±10-14 days). Used to confirm due date if 1st trimester wasn't done.
3rd trimester ultrasound: not reliable for dating; fetal sizes vary significantly.
Best practice: 1st trimester ultrasound is the gold standard. If the dating differs from LMP-based calculation by more than 7 days, the ultrasound date is usually used.
Standard milestones
Common milestones during pregnancy:
| Week (gestational) | Date relative to LMP | Milestone |
|--------------------|----------------------|-----------|
| 4-5 weeks | 28-35 days post-LMP | Pregnancy detectable on test |
| 6-9 weeks | 42-63 days | Embryo to fetus transition |
| 8-12 weeks | 56-84 days | First trimester ends; ultrasound dating |
| 12-16 weeks | 84-112 days | Optional first screening tests |
| 18-22 weeks | 126-154 days | Anatomy ultrasound |
| 22-24 weeks | 154-168 days | Possible viability if born |
| 28 weeks | 196 days | Glucose tolerance test |
| 32-36 weeks | 224-252 days | Group B strep screening |
| 37-42 weeks | 259-294 days | Term pregnancy; due date in this range |
40 weeks is the median. 90% of births occur between 37 and 42 weeks.
When the due date moves
The due date can change during pregnancy:
1st trimester ultrasound: refines from LMP estimate by ±5-7 days.
2nd or 3rd trimester ultrasound: rarely changes due date significantly.
Multiple pregnancy (twins, triplets): same gestational age but different growth patterns; often delivered earlier.
High-risk factors: preeclampsia, placenta previa, etc. — may schedule delivery earlier than due date.
Spontaneous early labor: when labor begins on its own before due date.
The due date is an estimate. Most births happen within ±2 weeks.
Common pregnancy math questions
"How many weeks pregnant am I?"
Subtract LMP from today, divide by 7. Or use a pregnancy calculator.
For someone with LMP March 1, 2026 and today is May 21, 2026:
- Days since LMP: 81
- Weeks pregnant: 81 / 7 = ~11.5 weeks
The duration calculator can compute this.
"When was I conceived?"
Approximately 14 days after LMP. For LMP March 1, conception was around March 15.
"When can I expect labor?"
Most women deliver between 37-42 weeks (gestational age). The due date is the median; specific timing varies.
"Can I be 'pregnant for 9 months'?"
The 9-month rule of thumb is approximate. Pregnancy is 40 weeks, which is closer to 9 months than 10. The exact length varies.
"Why is my baby's birthday different from due date?"
Due dates are estimates. Most births are 1-2 weeks earlier or later. Only ~5% of births occur exactly on the due date.
Calendar tools for pregnancy
Pregnant women often use:
Pregnancy apps: Track weeks, milestones, baby development.
Calendar with weekly markers: Visualize trimesters and milestones.
The duration calculator: Compute exact weeks/days pregnant on any date.
Ultrasound reports: Provide refined dating.
Special cases
IVF dates
In IVF, conception (fertilization) is precisely known. The convention shifts:
- "Day 0" can be fertilization (IVF) or LMP (natural)
- For IVF: conception date + 38 weeks = due date estimate
- Equivalent to: embryo transfer date + ~14 days = "LMP equivalent"
Most IVF clinics and hospitals adjust calculations accordingly.
Late pregnancy
Pregnancies extending past 42 weeks ("post-term") are rare but possible. Hospitals usually induce labor by 42 weeks for safety.
Miscarriage/loss
Loss before 20 weeks is termed miscarriage. After 20 weeks, stillbirth. The due date concept doesn't apply.
Multiple pregnancies
Twins typically deliver at 36-37 weeks. Triplets at 32-34 weeks. The "due date" is the planned scheduled delivery, not necessarily natural-term.
Working with healthcare providers
When discussing pregnancy with healthcare providers:
Have your LMP date ready. They'll calculate everything from it.
Bring ultrasound reports. They contain refined dating information.
Don't worry about exact precision in early conversations. ±2 weeks is normal in initial consultations.
Ask for clarification if calculations seem off. "Why is my due date different from what the calculator showed?" is a fair question.
FAQ
Is the due date precise?
No — it's an estimate. Only ~5% of births occur exactly on the due date. Most are within ±2 weeks.
Why count from LMP, not conception?
LMP is easier to identify. The convention dates back to before precise conception dating was possible. It's still standard medical practice.
Can ultrasound change my due date?
Yes — 1st trimester ultrasound is the gold standard for refining due date. Differences of more than 7 days from LMP-based calculation usually mean the ultrasound takes precedence.
What if my menstrual cycle is irregular?
The LMP-based calculation may be off by a week or more. Ultrasound dating is essential.
Should I trust the pregnancy app or my doctor?
Both. Apps use the same standard math, but your doctor accounts for medical history and ultrasound findings. Discuss with your doctor for accurate predictions.
What's "term" pregnancy?
Term: 37+ weeks gestational age. Early term: 37-38 weeks. Full term: 39-40 weeks. Late term: 41 weeks. Post-term: 42+ weeks.
Bottom line
Pregnancy due date is calculated as LMP + 280 days (40 weeks). It's a starting estimate; ultrasound dating refines it. Most births occur within ±2 weeks of the due date. Use the duration calculator to track your week-by-week progress between known dates.