Pregnancy due date and prenatal schedule calculator
This page follows the Korean due-date planner. It preserves the LMP, ovulation, IVF, ultrasound, planned delivery, multiple pregnancy, high-risk pregnancy, and prenatal schedule assumptions used by the source calculation.
Dating methods preserved from the Korean source
The default pregnancy length is 280 days from the last menstrual period. If the average cycle is not 28 days, the calculator adjusts the LMP due date by the cycle difference. Ovulation-based dating uses 266 days from ovulation.
For assisted reproduction, the Korean source uses 266 days from a day-3 embryo transfer and 261 days from a day-5 blastocyst transfer. Ultrasound dating starts from the ultrasound date and subtracts the gestational age already elapsed from the standard 280-day pregnancy period.
- LMP: last menstrual period plus 280 days, with cycle-length adjustment.
- Ovulation: ovulation date plus 266 days.
- IVF day 3: embryo transfer date plus 266 days.
- IVF day 5: embryo transfer date plus 261 days.
- Ultrasound: ultrasound date plus the remaining days to 280 days.
Priority when dates disagree
The Korean logic reconciles multiple candidate dates by priority. Planned delivery is treated as the strongest anchor, then ultrasound, IVF day-5, IVF day-3, ovulation, and finally LMP. Each candidate keeps a difference in days from the selected primary date so users can see why two methods disagree.
This matters because early ultrasound may differ from LMP when ovulation was late, cycles are irregular, or the LMP is uncertain. IVF dating is usually precise because the transfer day and embryo age are known.
Schedule adjustments
The source planner builds a prenatal schedule from the selected due date. Multiple pregnancy shifts the schedule anchor 14 days earlier, and high-risk pregnancy shifts it another 7 days earlier. These adjustments do not rewrite the biological due date; they move the planning window for follow-up care.
The summary divides pregnancy into trimesters: before 14 weeks is trimester 1, weeks 14 to 27 are trimester 2, and week 28 and later is trimester 3. Progress is calculated as elapsed gestational days divided by the effective gestation length.
- Multiple pregnancy planning adjustment: 14 days earlier.
- High-risk pregnancy planning adjustment: 7 days earlier.
- Trimester 1: under 14 weeks.
- Trimester 2: 14 weeks through 27 weeks.
- Trimester 3: 28 weeks and later.
Practical use
Use LMP when cycles are regular and the last period date is reliable. Use ultrasound when the clinician has provided a gestational age that conflicts with the menstrual estimate. Use IVF transfer dating after fertility treatment. Use planned delivery only when a cesarean or induction date is already set.
The result is a planning estimate for appointments, administrative dates, leave preparation, and calendar export. Final clinical dating should follow the obstetric clinician who has access to ultrasound, fetal growth, maternal history, and local hospital policy.