Method 1: Last Menstrual Period (LMP) — Most Common
The most widely used method for calculating a due date is based on the first day of your Last Menstrual Period (LMP). This method is used by virtually all healthcare providers as the starting point for pregnancy dating. It assumes a standard 28-day menstrual cycle with ovulation on day 14.
Why LMP and not conception date? Because the exact date of ovulation and fertilization is rarely known, while the LMP date is usually remembered or recorded. Pregnancy is conventionally counted from the LMP, making it approximately 2 weeks longer than actual fetal age (from conception).
Step-by-step: Write down the first day of your last period → Add 280 days (40 weeks) → That is your estimated due date (EDD).
Naegele's Rule — The Classic Formula
Naegele's Rule is the mathematical formula behind LMP-based due date calculation, developed by German obstetrician Franz Karl Naegele in 1812 and still in universal use today:
Naegele's Rule:
EDD = LMP + 1 year - 3 months + 7 days
Example: LMP = January 15, 2026
Step 1: Add 1 year → January 15, 2027
Step 2: Minus 3 months → October 15, 2026
Step 3: Add 7 days → October 22, 2026
Due Date = October 22, 2026
This formula is mathematically equivalent to adding 280 days (40 weeks) to your LMP date. Our Due Date Calculator does this instantly — just enter your LMP date and get your EDD, current gestational week, and trimester.
280Days from LMP to due date
40Weeks full-term pregnancy
4%Babies born on exact due date
37–42Weeks considered full term
Method 2: Known Conception Date
If you know the exact date of conception (e.g., from ovulation tracking, OPK strips, or fertility monitoring), you can calculate your due date more directly:
EDD = Conception Date + 266 days (38 weeks)
Example: Conception date = February 1, 2026
EDD = February 1 + 266 days = October 25, 2026
Note: 266 days from conception = 280 days from LMP
(assumes ovulation on day 14 of a 28-day cycle)
If your cycle is longer or shorter than 28 days, conception likely occurred on a different day relative to your LMP. For example, if your cycle is 35 days, ovulation likely occurred around day 21 — one week later than standard. Adjust accordingly by adding (cycle length - 28) days to your calculated EDD.
Method 3: Ultrasound Dating — Most Accurate
An early ultrasound (ideally performed between 7–13 weeks) provides the most accurate gestational age determination. The sonographer measures the Crown-Rump Length (CRL) — the distance from the top of the baby's head to its bottom — and compares it to established growth charts to calculate gestational age within ±5–7 days.
When should ultrasound dating override LMP dating? The ACOG (American College of Obstetricians and Gynecologists) recommends adjusting the EDD based on ultrasound if the discrepancy is:
- Before 9 weeks: More than 5 days difference
- 9–15 weeks 6 days: More than 7 days difference
- 16–21 weeks 6 days: More than 10 days difference
- 22 weeks or later: More than 14 days difference
Women with irregular menstrual cycles, unknown LMP, or PCOS often have more accurate dating from early ultrasound than from LMP calculation.
Method 4: IVF Transfer Date
For pregnancies achieved through In Vitro Fertilization (IVF), due date calculation is more precise because the exact date of fertilization and embryo age is known:
For Day 3 Embryo Transfer:
EDD = Transfer Date + 263 days
For Day 5 Embryo Transfer (Blastocyst):
EDD = Transfer Date + 261 days
Example: Day 5 blastocyst transferred on March 1, 2026
EDD = March 1 + 261 days = November 17, 2026
How Accurate Is Your Due Date?
Despite the precision of calculation, birth rarely happens on the estimated due date. Research consistently shows:
- Only about 4% of babies are born on their exact due date
- About 70% of births occur within 10 days of the EDD
- About 90% of births occur within 2 weeks of the EDD
- A "normal" delivery window is 37 weeks 0 days through 42 weeks 0 days
The due date is best understood as the center of a 5-week window (weeks 37–42) during which delivery is expected, not a precise deadline. Post-term pregnancies (beyond 42 weeks) are monitored carefully and often induced due to increasing placental insufficiency risk.
Key Pregnancy Milestones by Week
Week 4: Missed period — first positive pregnancy test
Week 6: Heartbeat detectable on transvaginal ultrasound
Week 8: All major organs forming; embryo becomes fetus
Week 12: End of first trimester; miscarriage risk drops significantly
Week 16: Gender may be visible on ultrasound
Week 18: Baby begins hearing sounds
Week 20: Anatomy scan ultrasound (checks for abnormalities)
Week 24: Viability threshold — survival possible with NICU support
Week 28: Third trimester begins; rapid brain development
Week 32: Baby practices breathing movements
Week 36: Baby considered "early term" — most systems mature
Week 39: Full term — optimal time for delivery
Week 40: Due date
Week 42: Post-term — induction typically discussed
Frequently Asked Questions
How do I calculate my due date from my last period?
Add 280 days (40 weeks) to the first day of your last menstrual period. Alternatively, use Naegele's Rule: take your LMP date, add 1 year, subtract 3 months, and add 7 days. For example, if your LMP was January 1, your due date would be approximately October 8. Our free Due Date Calculator does this instantly.
How accurate is a due date calculated from LMP?
LMP-based due dates are accurate within about ±2 weeks for women with regular 28-day cycles. For women with irregular cycles, PCOS, or unknown LMP, accuracy is lower. An early ultrasound (7–13 weeks) is more accurate and can refine the due date to within ±5–7 days. Most providers use LMP for initial dating and confirm with early ultrasound.
What if my due date changes after an ultrasound?
It is very common for due dates to be adjusted after an early ultrasound. If the ultrasound measurement differs from the LMP-based date by more than 5–7 days in the first trimester, your provider will typically revise the due date. The revised date based on early ultrasound is usually more accurate. This does not indicate anything is wrong with your pregnancy.
Can I go into labor before my due date?
Yes. Any birth between 37 weeks 0 days and 41 weeks 6 days is considered normal timing. Births at 37–38 weeks are called early term, 39–40 weeks are full term, 41 weeks are late term, and 42+ weeks are post-term. About 11% of births in the US occur before 37 weeks (preterm). Signs of labor include regular contractions, water breaking, and bloody show.