Conception Date Due Date Calculator
Enter your conception date to estimate your due date. Standard pregnancy length from conception is 266 days (38 weeks).
Medical note: this tool gives an estimate, not a diagnosis. Your provider may refine your due date using ultrasound and clinical history.
How this conception-based due date calculator works
Most pregnancy apps use the first day of your last menstrual period (LMP). This page is different: it starts from your estimated conception date. That can be useful if you tracked ovulation carefully, used fertility treatment, or know a likely conception window.
The default formula is simple: Due Date = Conception Date + 266 days. That equals 38 weeks from conception, which is the classic biological pregnancy length used in many medical references.
Why 266 days?
Pregnancy timing is usually discussed in two parallel ways:
- Gestational age (clinical): counted from LMP, typically around 280 days.
- Fetal age (conception-based): counted from fertilization, typically around 266 days.
Because ovulation and fertilization usually occur about two weeks after LMP in a 28-day cycle, these systems differ by about 14 days.
How accurate is a due date from conception?
A due date is an estimate, not a guarantee. Even with excellent cycle tracking, implantation timing and normal biological variation can shift delivery day.
Factors that can influence real delivery timing
- Natural variation in embryo development and implantation timing
- Uncertainty in exact ovulation/fertilization timing
- Cycle irregularity or hormonal shifts
- Maternal health factors and pregnancy complications
- Provider decision-making around induction or planned delivery
In practice, only a small percentage of births occur on the exact estimated due date. Many healthy births happen within a wider term window.
When a conception-based calculator is especially useful
- You tracked ovulation with LH tests, BBT, or ultrasound
- You conceived through timed intercourse with a narrow fertility window
- You had assisted reproduction and know a precise transfer schedule
- You want a second estimate to compare with an LMP-based app
Quick step-by-step guide
1) Enter your conception date
Choose the date you believe conception likely occurred.
2) Keep 266 days (or adjust if advised)
Most people should leave the default value of 266. Only change it if your clinician has asked you to use a specific estimate.
3) Click calculate
You’ll get an estimated due date plus useful milestone dates.
FAQ: due date calculator by conception date
Is this better than an LMP calculator?
Not always. It can be better when conception timing is known well. Otherwise, LMP and first-trimester ultrasound are often the best combined approach.
Can I use this if I have irregular cycles?
Yes, and conception-based methods may actually be more helpful in irregular cycles if ovulation was tracked clearly.
Should I worry if my ultrasound date is different?
Small differences are common. Your OB/GYN or midwife will use clinical standards to choose the most reliable due date for care planning.
Important reminder
This calculator is educational and planning-focused. It does not replace prenatal care. If you are pregnant or trying to conceive, schedule regular visits with a qualified healthcare professional.