LMP · Conception Date · IVF Transfer — week-by-week milestones
Medical note: Due dates are estimates — only about 5% of babies arrive on their calculated date. Ultrasound dating, especially in the first trimester, is more accurate than LMP-based calculations. This tool is for informational purposes only and is not a substitute for medical advice. Please consult your healthcare provider for personalized guidance.
The date your last period started
Typical range: 20–45 days (default 28)
Method: Naegele's rule — LMP + 280 days (adjusted for cycle length variation from 28-day baseline).
The date of ovulation or confirmed conception
Method: Conception offset — Conception date + 266 days (38 weeks). This is 280 days minus the typical 14-day lag from LMP to ovulation.
The date of embryo transfer
Method: IVF transfer offset — Day-5: transfer date + 261 days; Day-3: transfer date + 263 days. Each offset corrects for embryo developmental age at the time of transfer.
Estimated Due Date
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Days remaining
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Trimester
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Conceived around
Trimester Progress
T1
T2
T3
Wk 1–13Wk 14–27Wk 28–40
Key Milestones
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Week-by-Week Pregnancy Milestones
Developmental highlights, key screenings, and size comparisons — weeks 4 through 42. Your current week is highlighted.
How Due Date Calculation Works
Naegele's Rule (LMP Method)
The most widely used method, formalized by German obstetrician Franz Karl Naegele in 1812. The rule adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For cycles longer or shorter than 28 days, the calculation is adjusted: due date = LMP + 280 + (cycle length − 28) days.
Example: LMP = March 1, 2025; cycle = 28 days → Due date = December 6, 2025.
Conception / Ovulation Date Method
When the exact conception or ovulation date is known (e.g., from ovulation tests or monitored cycles), the due date is calculated by adding 266 days (38 weeks) to the conception date. This equals the LMP method's 280 days minus the 14-day presumed gap from period to ovulation.
IVF Transfer Date Method
For IVF pregnancies, the embryo's developmental age at transfer is known precisely:
Transfer type
Embryo age
Days added to transfer date
Equivalent LMP offset
Day-5 blastocyst
5 days
+261 days
Transfer date = LMP + 19 days
Day-3 cleavage
3 days
+263 days
Transfer date = LMP + 17 days
Worked example (IVF Day-5): Transfer on April 10, 2025 → due date = April 10 + 261 days = December 27, 2025.
Trimester Dates
Trimesters are measured in gestational weeks from the LMP date:
First trimester: Weeks 1–13 (day 1 through day 97 from LMP)
Second trimester: Weeks 14–27 (day 98 through day 195)
Third trimester: Weeks 28–40+ (day 196 onward)
Frequently Asked Questions
How is a due date calculated from LMP?
Using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a standard 28-day cycle; for other cycle lengths the ovulation offset is adjusted accordingly.
How is the due date calculated from a conception date?
Add 266 days (38 weeks) to the conception or ovulation date. This is because fertilization typically occurs around day 14 of a 28-day cycle — 280 − 14 = 266 days from conception to estimated delivery.
How is an IVF due date calculated?
For a Day-5 (blastocyst) transfer, add 261 days to the transfer date. For a Day-3 transfer, add 263 days. These offsets account for the embryo's developmental age at the time of transfer, back-calculating an equivalent LMP date.
When does each trimester start and end?
The first trimester runs from week 1 through the end of week 13. The second trimester is weeks 14–27. The third trimester begins at week 28 and continues through delivery (typically around week 40).
Is the due date an exact prediction?
No. Due dates are estimates — only about 5% of babies are born on their calculated date. Normal delivery ranges from 37 to 42 weeks. Ultrasound dating, especially in the first trimester (before 14 weeks), is more accurate than LMP-based calculations. Always consult your healthcare provider.
What is Naegele's rule?
Naegele's rule was published in 1812 by German obstetrician Franz Karl Naegele. It calculates the estimated due date by adding one calendar year, subtracting three months, and adding seven days to the LMP — equivalent to adding 280 days.