健康・フィットネス

妊娠計算機

出産予定日・妊娠週数・重要なマイルストーンを計算

使い方
  1. Enter the exact date of the First Day of your Last Period (LMP).
  2. Enter your Average Cycle Length. The standard average is 28 days.
  3. Click 出産予定日を計算 to generate your pregnancy timeline.

計算式

This calculator relies on a modified Naegele's Rule to estimate your due date:

  • Start with the first day of your LMP.
  • Add 280 days (40 weeks) for a standard 28-day cycle.
  • If your cycle is longer or shorter than 28 days, the due date is adjusted accordingly.

Disclaimer: Only about 5% of babies are born exactly on their estimated due date. Always consult your healthcare provider for medical confirmation.

How Is a Pregnancy Due Date Calculated? Naegele’s Rule Explained

The most widely used method for estimating a pregnancy due date is Naegele’s Rule, developed by German obstetrician Franz Karl Naegele in 1812. It calculates the Estimated Due Date (EDD) by adding 280 days (40 weeks) to the first day of the Last Menstrual Period (LMP), assuming a standard 28-day cycle and ovulation on day 14.

Our calculator applies a cycle-length adjustment: if your cycle is longer or shorter than 28 days, the due date shifts proportionally. For example, a woman with a 35-day cycle ovulates approximately 7 days later than the 28-day baseline, making her EDD approximately 7 days later. The Wikipedia article on estimated date of delivery provides an excellent overview of the various dating methods used in modern obstetrics.

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EDD (Standard): LMP + 280 days
EDD (Adjusted): LMP + 280 + (cycle length − 28) days
Only ~5% of babies arrive on their exact EDD. A normal delivery range is 37–42 weeks gestation.

Pregnancy Trimesters: A Week-by-Week Overview

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First Trimester (Weeks 1–13) — The most critical period for fetal organ development. Morning sickness, fatigue, and breast tenderness are common. Key milestones: heartbeat detectable at week 6, nuchal translucency screening at weeks 11–13. Neural tube closure (weeks 3–4) is why pre-conception folic acid is essential.
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Second Trimester (Weeks 14–26) — Often called the "honeymoon trimester" as early symptoms typically subside. The fetus develops hearing (week 18), and most parents learn the sex via anatomy scan (weeks 18–20). Quickening (first fetal movements) is felt around weeks 16–25.
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Third Trimester (Weeks 27–40+) — Rapid fetal weight gain. The fetus reaches viability (ability to survive outside the womb) at 24–28 weeks, though neonatal intensive care is required this early. Braxton Hicks contractions begin; the fetus moves into a head-down position for birth (weeks 32–36).

Prenatal Nutrition: Key Nutrients for a Healthy Pregnancy

Evidence-based nutritional priorities during pregnancy, according to the American College of Obstetricians and Gynecologists (ACOG):

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Folic Acid (400–800 mcg/day) — Prevents neural tube defects. Ideally started 1–3 months before conception and continued through the first trimester.
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Iron (27 mg/day) — Supports maternal blood volume expansion (+50%) and fetal iron stores. Deficiency causes anaemia, which is associated with preterm delivery and low birth weight.
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Calcium (1,000 mg/day) — Supports fetal bone and tooth development. If intake is insufficient, the fetus draws calcium from maternal bones, increasing osteoporosis risk later in life.
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Vitamin D (600 IU/day) — Essential for calcium absorption and fetal immune development. Many pregnant women are deficient, particularly in Northern climates. Supplementation is widely recommended.
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Omega-3 DHA (200–300 mg/day) — Critical for fetal brain and retinal development. Found in oily fish; supplementation recommended for those who do not consume fish regularly.

Frequently Asked Questions About Pregnancy Due Dates

How accurate is the due date calculator?

Calendar-based due date calculators are accurate to within ±1–2 weeks for most women with regular cycles. Ultrasound dating (especially in the first trimester, at 8–12 weeks) is more accurate—within ±5–7 days. If the LMP date and ultrasound date differ by more than 7 days, most obstetricians defer to the ultrasound date.

What is gestational age vs. fetal age?

Gestational age is counted from the first day of the LMP—approximately 2 weeks before conception. Fetal age (or embryonic age) is counted from actual conception. Because conception timing is rarely known precisely, gestational age is the standard used in obstetrics, making a "40-week pregnancy" actually about 38 weeks of fetal development.

What is preterm, term, and post-term pregnancy?

According to ACOG: Early preterm: <34 weeks. Late preterm: 34–36 weeks. Early term: 37–38 weeks. Full term: 39–40 weeks. Late term: 41 weeks. Post-term: 42+ weeks. "Full term" (39–40 weeks) is associated with the best outcomes; induction is typically offered by 41–42 weeks to avoid post-term complications.

When should I take a pregnancy test?

Most home pregnancy tests (HPTs) can reliably detect hCG from the first day of a missed period (approximately 14 days after conception). Some early-result tests claim accuracy from 5–6 days before the missed period, though sensitivity decreases the earlier you test. For the most reliable result, test the morning of your first missed period using first-morning urine.

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カロリー計算機Increased calorie needs during pregnancy (+300 kcal/day in 2nd–3rd trimester)
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理想体重計算機Pre-pregnancy weight range by height

References & Further Reading

  1. Wikipedia contributors. Estimated Date of Delivery. Wikipedia, The Free Encyclopedia.
  2. Wikipedia contributors. Pregnancy — Signs, Stages, and Complications. Wikipedia, The Free Encyclopedia.
  3. ACOG. Nutrition During Pregnancy. American College of Obstetricians and Gynecologists, 2023.
  4. Mayo Clinic. Pregnancy Week by Week. Mayo Clinic, 2024.
  5. NHS. Due Date Calculator and Pregnancy Information. National Health Service, 2023.