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Pregnancy Due Date Calculator

Estimates the due date, conception date and current gestational age from the first day of the last menstrual period, adjusting for cycle length (Naegele's rule).

When to use

You know the first day of the last period and want the estimated due date, trimester dates or gestational age on a given date.

Do not use when: Cycles are very irregular, the LMP is unknown, or an ultrasound dating is available (ultrasound dating in the first trimester is more accurate and takes precedence).

Formula

due_date = LMP + 280 days + (cycle_length − 28); conception ≈ LMP + 14 + (cycle_length − 28); gestational_age = as_of_date − LMP

Inputs

Parameter	Type	Unit	Required	Description
`last_period_date`	date		yes	First day of the last menstrual period (ISO date YYYY-MM-DD).
`cycle_length_days`	integer	days	default 28	Average menstrual cycle length; the due date shifts by (cycle − 28) days. Range: ≥ 20, ≤ 45
`as_of_date`	date		no	Optional date on which to report gestational age (weeks + days). Omit to skip.

Outputs

Output	Type	Unit	Description
`due_date`	date		LMP + 280 days + (cycle − 28).
`conception_date_estimate`	date		Approximately LMP + 14 days + (cycle − 28).
`second_trimester_start`	date		Week 14 (day 91 of pregnancy).
`third_trimester_start`	date		Week 28 (day 189).
`gestational_age`	string		Weeks and days as of as_of_date (only when as_of_date is given).
`gestational_days`	integer	days	Days since LMP as of as_of_date (only when given).

Example

LMP 2026-01-01, 28-day cycle, as of 2026-04-15: {"last_period_date":"2026-01-01","cycle_length_days":28,"as_of_date":"2026-04-15"} → {"due_date":"2026-10-08","conception_date_estimate":"2026-01-15","second_trimester_start":"2026-04-02","third_trimester_start":"2026-07-09","gestational_age":"14 weeks 6 days","gestational_days":104}

LMP 2026-03-10, 32-day cycle: {"last_period_date":"2026-03-10","cycle_length_days":32} → {"due_date":"2026-12-19"}

GET https://tttkmbb.com/api/v1/calculate/pregnancy-due-date?last_period_date=2026-01-01&cycle_length_days=28&as_of_date=2026-04-15

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/pregnancy-due-date (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/pregnancy-due-date · Markdown: https://tttkmbb.com/health/pregnancy-due-date.md · JSON definition: https://tttkmbb.com/health/pregnancy-due-date.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="pregnancy-due-date"
OpenAPI operationId: calculate_pregnancy_due_date
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

ACOG Committee Opinion 700 (2017) Methods for Estimating the Due Date (standard)

FAQ

How accurate is the due date?

Only about 4% of babies arrive on the estimated date; most are born within two weeks either side. First-trimester ultrasound refines the estimate.

How are trimesters defined?

Following ACOG: first trimester through 13 weeks 6 days, second from 14 weeks 0 days, third from 28 weeks 0 days.

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