Last Period Due Date Calculator

Calculator inputs

Formula used

The estimate is based on a cycle-adjusted version of the standard last-period method:

• Estimated Due Date (EDD) = LMP + 280 days + (CycleLength − 28)
• Estimated Ovulation Day ≈ CycleLength − 14 (or your provided ovulation day)
• Gestational Age Today = Today − LMP

The cycle adjustment shifts the estimate later for longer cycles and earlier for shorter cycles.

How to use this calculator

1. Enter the first day of your last menstrual period.
2. Set your average cycle length using your recent history.
3. If you track ovulation, add the cycle day to refine timing.
4. Press Submit to view results above the form.
5. Use Download CSV or Download PDF to save results.

Example data table

Example rows are illustrative. Your results depend on your inputs.

Why last period dating is widely used

Pregnancy length is commonly counted from the first day of the last menstrual period, not from conception. This convention produces a full-term estimate of about 280 days or 40 weeks. The approach works well when cycles are regular and early ultrasound is unavailable.

Cycle length adjustment and its effect

A 28‑day cycle is the baseline. If your average cycle is longer or shorter, the estimate shifts by (cycle length − 28) days. For example, a 32‑day cycle moves the estimate about 4 days later, while a 24‑day cycle moves it about 4 days earlier.

Gestational age and trimester ranges

Gestational age is the time from your last period to today. Trimesters are often grouped as 0–13 weeks, 14–27 weeks, and 28–40 weeks. The calculator reports weeks and days to help align appointments and symptom tracking.

Key milestone windows shown by the tool

Typical timing includes the anatomy scan around 18–22 weeks and glucose screening around 24–28 weeks. A commonly cited viability milestone appears near 24 weeks. Full term is often considered from about 37 weeks, while post‑term is usually after 42 weeks.

When ultrasound dating may differ

Ovulation and implantation can vary, especially with irregular cycles, recent postpartum changes, or hormonal contraception. Early ultrasound dating may refine the estimate, particularly when performed in the first trimester. Differences of several days are common and usually normal.

Using exports for planning and communication

The CSV export is useful for personal tracking and spreadsheets, while the PDF export supports printing or sharing. Share only what you’re comfortable with; the output includes dates and milestones, not sensitive health history. Bring your estimate to prenatal visits for context, then follow clinical guidance.

FAQs

Is the due date exact?

It’s an estimate. Many births occur within a two‑week range around the calculated date, and ultrasound or clinical findings may adjust timing.

What if my cycles are irregular?

Use your best average cycle length. Irregular cycles can shift ovulation, so consider confirming dates with early ultrasound if recommended.

Why count from the last period instead of conception?

Conception date is often uncertain. Counting from the last period standardizes timelines and produces the common 40‑week gestational length.

Can I enter an ovulation day?

Yes. If you track ovulation, entering a cycle day refines the ovulation and conception estimate, while the due date remains LMP‑based with cycle adjustment.

What if I don’t remember the exact LMP date?

Choose the closest likely start date. If you’re unsure, early prenatal assessment can help establish a more reliable gestational age.

Should I rely on this tool for medical decisions?

No. Use it for planning only. For symptoms, testing, or care decisions, follow guidance from your clinician or local health services.