What are the three trimesters?

First trimester (weeks 1-12): major organ formation, morning sickness common. Second trimester (weeks 13-27): rapid growth, movement felt, often the most comfortable period. Third trimester (weeks 28-40): final development, weight gain, preparation for delivery.

Does cycle length affect the due date?

Yes. Naegele's Rule assumes a 28-day cycle. If your cycle is 35 days, you likely ovulated 7 days later, pushing the due date out by one week. Adjust by adding the difference between your cycle length and 28 days.

What is gestational age versus fetal age?

Gestational age counts from the first day of the LMP (about 2 weeks before conception). Fetal age counts from actual conception. Gestational age is always about 2 weeks longer and is the standard used in medical settings.

When should I expect my first prenatal visit?

Most healthcare providers schedule the first prenatal visit around 8-10 weeks gestational age. This visit typically includes a health history, physical exam, blood tests, and often the first ultrasound to confirm the pregnancy and estimate due date.

Pregnancy Calculator

Find your due date, gestational age in weeks, and trimester — from your last menstrual period or conception date.

Pregnancy Calculator

Estimate pregnancy milestones

Pregnancy Due Date

Enter the first day of your last menstrual period

Last Menstrual Period (YYYY-MM-DD)

Formula

Due Date = LMP + 280 days (Naegele's rule)

What Is a Pregnancy Calculator?

A pregnancy calculator estimates your due date based on Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This standard medical formula has been used for over a century. It assumes a 28-day cycle and places ovulation at day 14. If your cycle is longer or shorter, the tool adjusts the due date accordingly, giving you a personalized estimate rather than a textbook average.

Pregnancy is divided into three trimesters: the first (weeks 1–12), second (weeks 13–26), and third (weeks 27–40). Each trimester marks distinct developmental milestones — from implantation and organ formation in the first trimester to rapid growth and lung maturation in the third. Keep in mind that only about 5% of babies arrive on their predicted due date. Most births happen within two weeks before or after that date, which is completely normal.

How to Use This Calculator

1Enter the first day of your last menstrual period (LMP) — or switch to conception date if you know it.
2Enter your average cycle length if it differs from 28 days (typical range is 21–35 days).
3Click Calculate to instantly see your estimated due date, current gestational age in weeks and days, and which trimester you're in.
4Review the weekly breakdown to track upcoming milestones throughout your pregnancy.

Formula & Method

Naegele Rule (from LMP):
  Due Date = LMP + 280 days (40 weeks)
  Due Date = LMP + 9 months + 7 days

From conception date:
  Due Date = Conception date + 266 days (38 weeks)

Gestational age (weeks):
  Weeks = (Today − LMP) / 7

Trimesters:
  1st: Weeks 1–12  (LMP to week 12)
  2nd: Weeks 13–26
  3rd: Weeks 27–40

Gestational age is always counted from the LMP, not from the conception date — which is why it's typically 2 weeks ahead of fetal age. A full-term pregnancy spans 37–42 weeks. This calculator is for informational purposes only; always consult your healthcare provider for medical guidance.

Worked Examples

Example 1 — LMP January 1, 28-day cycle

LMP = January 1. Using Naegele's Rule: add 280 days → Due Date = October 8. If today is March 1, the gestational age is 59 days ÷ 7 = 8 weeks 4 days — solidly in the 1st trimester. The baby's heart is already beating and major organs are beginning to form.

Example 2 — LMP March 15

LMP = March 15 → Due Date = December 20. By June 1 (78 days later), gestational age = 78 ÷ 7 = 11 weeks 1 day — approaching the end of the 1st trimester. The nuchal translucency ultrasound is typically scheduled around this time (weeks 11–14).

Example 3 — Known conception date February 14

Conception Date = February 14. Due Date = February 14 + 266 days = November 6. Since gestational age counts from LMP (≈ 14 days before conception), the estimated LMP = January 31. On May 1, gestational age = (May 1 − Jan 31) = 90 days ÷ 7 = 12 weeks 6 days — just at the boundary of the 1st and 2nd trimester.

Frequently Asked Questions

How accurate is the due date calculation?▾

The due date is an estimate, not a guaranteed delivery date. Naegele's Rule is accurate to within ±2 weeks for most pregnancies with regular 28-day cycles. An early ultrasound (before 13 weeks) is more accurate than LMP dating and is often used to confirm or revise the estimated due date. Factors like irregular cycles, late ovulation, and IVF dating can all affect accuracy.

What is the difference between gestational age and fetal age?▾

Gestational age is measured from the first day of the last menstrual period (LMP) and is the standard used by healthcare providers. Fetal age (also called embryonic age) is measured from the date of fertilization/conception — roughly 2 weeks less than gestational age. When your doctor says you're 10 weeks pregnant, that's gestational age; the embryo itself is about 8 weeks old.

What are the key milestones by trimester?▾

1st trimester (weeks 1–12): implantation, embryo forms, heartbeat detectable around week 6, organs begin forming, risk of miscarriage highest. 2nd trimester (weeks 13–26): rapid growth, movement felt (quickening) around weeks 18–22, anatomy scan at 20 weeks, risk of miscarriage drops significantly. 3rd trimester (weeks 27–40): lung maturation, brain development, baby gains weight rapidly, Braxton Hicks contractions begin.

Can the due date change after an ultrasound?▾

Yes, and it's common. If the ultrasound-based gestational age differs from the LMP-based estimate by more than 5–7 days (in the first trimester) or more than 10–14 days (in the second trimester), providers typically revise the due date to match the ultrasound measurement. This is because early crown-rump length measurements are very precise.

What is a post-term pregnancy?▾

A pregnancy is considered post-term when it extends past 42 completed weeks (294 days from LMP). At that point, most providers recommend induction due to increased risks of placental insufficiency and fetal complications. Between weeks 41 and 42, careful monitoring is standard. Roughly 10% of pregnancies reach 42 weeks, so while uncommon, it's not rare.