Estimated Due Date
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  • Gestation{{ days }} days ({{ weeks }} wks)
  • Remaining{{ daysLeft }} days
  • {{ t.label }}{{ t.start }} – {{ t.end }}
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MilestoneDate
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Categories: Calculator , Health & Fitness
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Gestational age counts the days from fertilisation (or the last menstrual period) to birth, letting clinicians and expectant parents plan timely check-ups, screenings, and support. Estimating the expected delivery date places every pregnancy event on a shared timeline and anchors trimester definitions.

This calculator accepts either the first day of the last period or a known conception date and, when needed, adjusts for non-standard cycle length. A lightweight reactive engine adds precisely 280 or 266 days, updates progress bars, and maps each week to milestones and fetal-growth medians.

Use it to schedule ultrasounds, book parental leave, and share realistic expectations with partners and caregivers. This calculator offers informational estimates, not medical advice.

Technical Details:

Expected delivery is traditionally calculated using Naegele’s rule: add 280 days to the first day of the last menstrual period, assuming a 28-day cycle and ovulation on day 14. When ovulation is tracked directly, adding 266 days to the conception date yields the same term length. Both methods generate a 40-week reference frame that guides prenatal care and public-health statistics.

Core formulas:

E=LMP+280−(28−c) E=C+266

E – estimated due date; LMP – first day of last period; C – conception date; c – cycle length in days.

BandWeeks (w)Description
First Trimester0 – 12 wOrganogenesis and early screening window
Second Trimester13 – 27 wRapid growth; anatomy scan and glucose test
Third Trimester28 – 40 wViability threshold, maturation, full term

Key parameters:

  • method – calculation basis (last period or conception).
  • start – reference date (YYYY-MM-DD).
  • cycle length – menstrual-cycle duration in days, 10–60 accepted.
  • unit toggle – metric versus imperial for fetal size display.

Example (LMP = 15 March 2025, cycle = 30 days):

280−(28−30)=282 15 Mar 2025+282 days=22 Dec 2025

Assumptions & limitations:

  • Assumes ovulation occurs exactly 14 days before next period.
  • Ignores ultrasonic re-dating after first-trimester scans.
  • Cycle-length adjustment presumes linear shift in ovulation timing.
  • Multiple gestations often deliver earlier than 40 weeks.

Edge cases & error sources:

  • Irregular or anovulatory cycles.
  • Implantation bleeding misidentified as LMP.
  • Leap-year calculations spanning 29 February.
  • Conception from assisted reproduction with embryo-transfer timing.

Scientific validity & references:

Naegele (1812) formula; ACOG Practice Bulletin #700 (Dating of Pregnancy, 2017); WHO fetal-growth standards (2021).

This browser-only calculator performs all computations locally and complies with GDPR regarding personal-health input.

Step-by-Step Guide:

Follow these quick steps to generate a personalised pregnancy timeline.

  1. Select Last period or Conception date.
  2. Enter the relevant date in the calendar field.
  3. Adjust Cycle length if it differs from 28 days.
  4. Review the summary card and switch tabs for milestones or fetal size.
  5. Download the CSV timeline for doctor visits or spreadsheet planning.

FAQ:

Why two methods?

Use the last-period method when ovulation is unknown; choose conception when the exact fertilisation date or embryo-transfer day is documented.

Cycle length too short?

The algorithm accepts 10–60 days. Values outside typical biologic limits produce less reliable dates and should be confirmed by ultrasound.

Is my data stored?

No. All inputs remain in your browser and vanish when the tab closes.

Can dates change?

Healthcare providers may revise the due date after early ultrasound measurements, which often supersede calendar estimates.

What about multiples?

Twins and higher-order pregnancies typically deliver before 40 weeks; subtract one to three weeks for planning purposes.

Glossary:

Gestational age
Time elapsed since fertilisation or LMP.
LMP
First day of the last menstrual period.
Trimester
One-third segment of the 40-week term.
EGA
Estimated gestational age, expressed in weeks and days.
Viability
Earliest gestation where survival outside womb becomes possible.