Calculator
Choose an input method, fill in dates, then press Calculate.
Example data table
| # | LMP | Cycle length | Reference date | Estimated EDD | Current trimester |
|---|---|---|---|---|---|
| 1 | 2025-12-10 | 28 days | 2026-02-01 | 2026-09-16 | First |
| 2 | 2025-11-20 | 30 days | 2026-03-05 | 2026-08-29 | Second |
| 3 | 2025-10-01 | 26 days | 2026-02-26 | 2026-07-06 | Second |
Formula used
- Cycle adjustment: adj = cycleLength − 28 (days).
- Estimated due date: EDD = LMP + (pregnancyWeeks × 7) + adj.
- LMP from due date: LMP = EDD − (pregnancyWeeks × 7) − adj.
- Conception estimate: LMP + (cycleLength − 14) days (rough ovulation estimate).
- Trimester boundaries: week-based ranges converted to dates using weeks × 7 days.
How to use this calculator
- Select whether you want to start from LMP or EDD.
- Enter the relevant date, then set your average cycle length.
- Pick a reference date to see progress for that day.
- Optionally adjust trimester week cutoffs for your preferred definition.
- Press Calculate to view results above the form.
- Use Download CSV or Download PDF for sharing.
Dating inputs that matter
This calculator works from the first day of the last menstrual period or an estimated due date. It also uses a reference date to show “where you are” on any day. A typical pregnancy length is 40 weeks, which equals 280 days, and you can model other common ranges between 37 and 44 weeks for planning. Entering the wrong date by one day shifts every milestone by one day.
Cycle length adjustment in practice
Cycle length changes the estimate by shifting the due date in days. The adjustment is cycleLength − 28, so a 30‑day cycle moves the timeline 2 days later, while a 26‑day cycle moves it 2 days earlier. This keeps the estimate consistent with later or earlier ovulation patterns used in routine scheduling.
Trimester boundaries and week mapping
Trimester dates come from week cutoffs converted into days. The default mapping is weeks 1–13, 14–27, and 28–40, but the cutoffs are editable because clinics may define them slightly differently. End dates are treated as inclusive, so week‑based plans, scans, and classes can be placed on clear calendar boundaries.
Gestational age and progress indicators
Gestational age is calculated as the number of days since the LMP, then expressed as weeks and days. The progress bar uses totalPregnancyDays as the denominator, and percentComplete is capped between 0 and 100 for readability. Days remaining to the estimated due date are shown to support appointment pacing and late‑pregnancy preparation. Reference date lets you compare past scans with today’s schedule in seconds easily.
Milestone planning with dated checkpoints
The output highlights practical checkpoints, including the end of the first trimester, a 24‑week milestone, the start of the third trimester, and the estimated due date. These dates can help structure nutrition reviews, movement tracking, and birth‑plan discussions. You can download results as CSV or PDF to share in visits or save in a folder.
Quality checks and clinical confirmation
All date calculations are planning estimates and should be validated against clinician guidance, especially with irregular cycles, recent contraception, breastfeeding, assisted conception, or early ultrasound dating. If your due date is clinically assigned, use that method for consistency. For symptoms such as bleeding, severe pain, or reduced movement, seek urgent care.
FAQs
What is LMP and why is it used?
Pregnancy dating commonly counts from the first day of your last menstrual period. It is easy to recall compared with ovulation day. The calculator converts that date into weeks, trimester ranges, and an estimated due date for planning.
How does cycle length change the result?
If your cycle is longer than 28 days, ovulation often occurs later, so the estimate shifts later by cycleLength − 28 days. Shorter cycles shift earlier. This adjustment moves trimester boundaries and milestones by the same number of days.
Can I calculate from an estimated due date instead?
Yes. Select the due date method and enter the due date. The calculator back-calculates an estimated LMP and then generates trimester dates, gestational age for the reference day, and milestone checkpoints for your schedule.
What should I enter for the reference date?
Use today’s date to see your current trimester and progress. You can also choose a past or future date to compare scan appointments, planned travel, or class timelines. The output updates gestational age, percent complete, and days remaining.
Why might my clinician’s dates differ?
Clinicians may revise dating based on early ultrasound, known conception timing, or fertility treatment. Different trimester cutoffs are also used in some settings. Use clinically assigned due dates when provided, and treat this tool as a planning aid.
Is this calculator safe to rely on for medical decisions?
No. It provides date estimates for organization and education. For symptoms such as bleeding, severe pain, fever, or reduced fetal movement, contact a healthcare professional promptly. Always follow individualized advice from your care team.