Due Date Calculator 2025

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First Day of Last Period

Average Cycle Length (days)

Your Due Date Results

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What is a Due Date Calculator?

A Due Date Calculator estimates when your baby will be born based on your last menstrual period (LMP), conception date, ultrasound results, or IVF embryo transfer date. The due date is calculated as 40 weeks (280 days) from the first day of your last period.

Our calculator provides 4 calculation methods for maximum accuracy, tracks your pregnancy week-by-week, shows trimester breakdowns, predicts baby's zodiac sign, identifies birth season, and provides full-term delivery timeline (37-42 weeks).

How to Use

1Select your calculation method (LMP, Conception, Ultrasound, or IVF)
2Enter the relevant date (last period, conception, ultrasound, or transfer)
3For LMP: Add your average cycle length if it's not 28 days
4For Ultrasound: Enter gestational age in weeks & days
5Get instant due date, current week, trimester & zodiac sign

4 Due Date Calculation Methods

📅

Last Menstrual Period (LMP)

Most Common: Calculates due date from first day of your last period using Naegele's Rule.

Naegele's Rule Formula:

Due Date = LMP + 280 days (40 weeks)

Adjusted if cycle ≠ 28 days

✓ Accuracy: ±2 weeks

✓ Best for: Regular 28-day cycles

⚠️ Limitation: Assumes ovulation on day 14

💫

Conception Date

Most Accurate: If you know exact conception/ovulation date.

Formula:

Due Date = Conception + 266 days (38 weeks)

From fertilization to birth

✓ Accuracy: ±1 week

✓ Best for: Known ovulation date

⚠️ Limitation: Difficult to know exact date

🔬

Ultrasound Dating

Doctor's Standard: Based on baby's measurements from ultrasound.

Formula:

Due Date = Ultrasound Date + (280 - Gestational Age in Days)

✓ Accuracy: ±5-7 days (first trimester)

✓ Best for: Most reliable method

⚠️ Limitation: Requires ultrasound scan

🧬

IVF Transfer Date

IVF Pregnancies: Precise calculation based on embryo transfer.

Formula:

3-day embryo: Transfer + 263 days

5-day blastocyst: Transfer + 261 days

✓ Accuracy: Very high (exact fertilization known)

✓ Best for: IVF pregnancies

Understanding Full-Term Pregnancy

While 40 weeks is the "due date," full-term pregnancy actually includes a range of weeks. Only 5% of babies arrive exactly on the due date. Here's what "full-term" really means:

⚠️

Preterm

Before 37 weeks

Premature birth, may need NICU care

🟡

Early Term

37-38 weeks

Baby's lungs & brain still developing

✅

Full Term

39-40 weeks

Optimal development, safest delivery time

🟠

Late/Post Term

41-42+ weeks

May require induction, increased monitoring

💡 Important: Most babies (90%) are born between 37-42 weeks. Doctors typically allow pregnancy to continue until 41-42 weeks before considering induction.

Why Do Due Dates Sometimes Change?

Your due date may be adjusted during pregnancy, especially after an ultrasound. Here's why:

Common Reasons for Date Changes:

First trimester ultrasound: More accurate than LMP method (becomes the "official" due date)
Irregular cycles: If your cycle isn't 28 days, LMP calculation is less accurate
Uncertain LMP date: If you can't remember exact date of last period
Baby's growth: Ultrasound measurements show baby is larger/smaller than expected
Multiple calculations differ: LMP and ultrasound dates don't match (ultrasound takes priority)

When Due Date Won't Change:

After 20 weeks: Second/third trimester ultrasounds less accurate for dating
IVF pregnancies: Transfer date is precise, won't be changed
Small measurement differences: Within 5-7 days of original due date (considered normal variation)
Doctor's policy: Most doctors don't change dates after first trimester unless significant discrepancy

Complete Pregnancy Timeline (40 Weeks)

Stage	Weeks	Key Developments
First Trimester	1-12	Conception, major organs form, heartbeat starts (week 5-6), morning sickness common
Second Trimester	13-26	Rapid growth, gender visible (week 16), mom feels kicks (week 18-20), viability at week 24
Third Trimester	27-40	Final growth, lungs mature, baby moves into position, full term at week 37

Frequently Asked Questions

1. How accurate is a due date calculator?

Due date calculators are approximately 80% accurate within 2 weeks of the predicted date. Only about 5% of babies arrive on their exact due date. Most babies (90%) are born between 37-42 weeks.

Accuracy by Method:

First Trimester Ultrasound: Most accurate±5-7 days

LMP Method: Standard calculation±2 weeks

IVF Transfer: Exact fertilization knownVery high

2. What if my baby doesn't come by the due date?

This is completely normal! Only 5% of babies arrive on their due date. Most doctors allow pregnancy to continue until 41-42 weeks before considering induction.

What Happens:

40 weeks (due date): Continue monitoring, most babies come within next 2 weeks
41 weeks: Increased monitoring (NST, ultrasounds), discuss induction options
42 weeks: Most doctors recommend induction to avoid complications
42+ weeks: Post-term pregnancy, induction strongly recommended

Note: Going past your due date is called "post-term" or "post-dates" pregnancy. Your doctor will monitor you closely and may recommend induction if pregnancy extends beyond 42 weeks.

3. Can my due date change during pregnancy?

Yes, your due date may be adjusted after a first-trimester ultrasound, which is more accurate than the LMP method. However, due dates typically don't change after 20 weeks.

When Due Dates Change:

✅ Will Change:

First trimester ultrasound shows different date than LMP (ultrasound becomes official date)
Difference of more than 7 days between LMP and ultrasound
Irregular cycles make LMP unreliable

❌ Won't Change: