Calculate your estimated due date, compare 4 medical methods, and track your pregnancy journey with a visual trimester timeline.
Calculation Method
20 days45 days
Weeks
Days (0-6)
Time Until Due Date
0months
0weeks
0days
Estimated Due Date
Enter your information
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Gestational Age
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Trimester
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Days Remaining
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Conception Est.
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Ovulation Est.
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% Complete
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Pregnancy Progress
1st Trimester (1-12 wks)
2nd Trimester (13-26 wks)
3rd Trimester (27-40 wks)
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LMP + 280 days (Naegele)± Cycle AdjustmentConception + 266 days
Pregnancy Milestones by Week
Week
Est. Date
Milestone
Baby Size
Status
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Prenatal Appointment Schedule
Estimated dates based on your due date. Always follow your provider's specific schedule.
Appointment
Week
Est. Date
Purpose
Enter due date above
4 Due Date Calculation Methods Compared
Different methods can produce different estimated dates. Enter your LMP date in the Calculator tab to see all 4 methods compared side by side.
Standard
Naegele's Rule
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LMP + 280 days
The most widely used method, assumes a 28-day cycle with ovulation on day 14. Used since 1812.
Research
Mittendorf-Williams
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LMP + 288 days (first) / 283 days (subsequent)
Based on a 1990 study; adjusts for parity. First pregnancies average 288 days, subsequent average 283 days.
Cycle-Adjusted
Parikh's Formula
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LMP + 9 months - 21 days + previous cycle length
Accounts for individual cycle variation. Most useful when cycles differ significantly from 28 days.
Gold Standard
Ultrasound Dating
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Based on fetal measurements (CRL)
Most accurate when done at 8-12 weeks (within 3-5 days). Accuracy decreases in later trimesters.
Due Date Range by Method
Recommendation: Enter your dates in the Calculator tab to see personalized results.
Method Reliability Comparison
Method
Accuracy
Best Used When
Limitation
Naegele's Rule
± 2 weeks
Regular 28-day cycles
Assumes ovulation on day 14
Mittendorf-Williams
± 2 weeks
Adjusting for first vs. subsequent pregnancy
Based on limited study population
Parikh's Formula
± 1-2 weeks
Irregular or non-28-day cycles
Requires known previous cycle length
Ultrasound (T1)
± 3-5 days
8-12 week dating scan
Less accurate after first trimester
Your Pregnancy Journey
A visual timeline of your pregnancy from conception to delivery, showing key milestones color-coded by trimester.
You Are Here
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Enter your dates to see your current position
Trimester Overview
Key Milestone Timeline
First Trimester Weeks 1-12
Week 4 — Embryo implants
Week 6 — First heartbeat detectable
Week 8 — All major organs forming
Week 10 — Officially a fetus
Week 12 — End of first trimester; miscarriage risk drops
Second Trimester Weeks 13-26
Week 14 — Gender may be visible
Week 18 — Anatomy scan window opens
Week 20 — Halfway point / anatomy scan
Week 22 — Baby can hear sounds
Week 24 — Viability milestone
Third Trimester Weeks 27-40
Week 28 — Brain developing rapidly
Week 32 — Major development complete
Week 36 — Weekly visits begin; baby dropping
Week 37 — Early term
Week 39 — Full term / optimal birth window
Week 40 — Estimated due date
How is a due date calculated?
A due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is called Naegele's rule. Only about 5% of babies are born on their exact due date; most arrive within 2 weeks before or after.
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How to Use This Calculator
1
Enter LMP or Conception Date
Input the first day of your last period or known conception/IVF transfer date.
2
Compare Methods
Switch to the Method Comparison tab to see how Naegele, Mittendorf-Williams, Parikh, and ultrasound dating differ.
3
Track Your Timeline
View the Trimester Timeline tab for a visual journey with key milestones, appointments, and your current position.
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Frequently Asked Questions
Basics How accurate is a due date calculated from my last period?
An LMP-based due date assumes ovulation on day 14 of a 28-day cycle. If your cycle is irregular or longer/shorter than 28 days, the estimate may be off by several days. A first-trimester ultrasound (8-12 weeks) is accurate to within 3-5 days and is considered the gold standard for dating.
Basics What percentage of babies are born on their due date?
Only about 4-5% of babies arrive on their exact due date. The normal delivery window spans 37-42 weeks of gestation. Most babies arrive within 1-2 weeks of the due date. First-time mothers tend to deliver slightly later than the due date on average.
Basics Can my due date change after an ultrasound?
Yes. If a first-trimester ultrasound differs from the LMP-based date by more than 5-7 days, most providers will adjust the due date to match the ultrasound. Later ultrasounds are less accurate for dating because fetal growth rates vary more in the second and third trimesters.
Basics What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period (LMP), which is about 2 weeks before conception. Fetal age (also called embryonic age) counts from the actual date of conception. A pregnancy described as 10 weeks gestational age is approximately 8 weeks fetal age.
Basics Which due date calculation method is most accurate?
A first-trimester ultrasound (8-12 weeks) is the most accurate single method, within 3-5 days. Among formula-based methods, Parikh's formula tends to be most accurate for women with irregular cycles since it accounts for individual cycle length. Mittendorf-Williams is useful for distinguishing first vs. subsequent pregnancies.
Advanced When is a pregnancy considered full term?
Full term is 39 weeks 0 days through 40 weeks 6 days. Early term is 37-38 weeks 6 days. Late term is 41 weeks 0 days through 41 weeks 6 days. Post-term is 42 weeks and beyond. Most providers will discuss induction between 39-41 weeks depending on individual circumstances.
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Formula & Methodology
Naegele's Rule
Due Date = LMP + 280 days (40 weeks)
The standard obstetric calculation. Add 7 days to LMP, subtract 3 months, add 1 year.
Mittendorf-Williams Rule
Due Date = LMP + 288 days (first pregnancy) or LMP + 283 days (subsequent)
Based on a 1990 Boston study that found first-time mothers carry 8 days longer than Naegele predicts.
Parikh's Formula
Due Date = LMP + 9 months - 21 days + previous cycle length
Accounts for individual cycle variation. Most accurate when previous cycle lengths are consistently known.
From Conception
Due Date = Conception Date + 266 days (38 weeks)
If conception date is known, pregnancy lasts 38 weeks from fertilization.
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Key Terms Explained
EDD — Estimated Due Date - the projected date of delivery, typically 40 weeks from the last menstrual period.
Gestational Age — Pregnancy age counted from the first day of the last menstrual period, about 2 weeks more than actual fetal age.
Trimester — Pregnancy divided into three trimesters: 1st (weeks 1-12), 2nd (weeks 13-27), 3rd (weeks 28-40).
Full Term — A pregnancy that reaches 39-40 weeks. Early term is 37-38 weeks. Post-term is after 42 weeks.
CRL — Crown-Rump Length - the measurement from the top of the head to the bottom of the buttocks, used in first-trimester ultrasound dating.
Nulliparous — A woman who has never given birth. First pregnancies tend to last slightly longer than subsequent ones.
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Real-World Examples
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Sarah, 29 — Regular 28-Day Cycle
Using LMP method to calculate due date, trimester boundaries, and current gestational age
Last Period (LMP)
Jan 6, 2025
Cycle Length
28 days
Method
Naegele’s Rule
Gestational Age (today)
~11 weeks 4 days
Estimated Due Date
Oct 12, 2025 — Libra ♎
1st Trimester ends
Apr 7, 2025
2nd Trimester ends
Jul 7, 2025
Sarah's LMP plus 280 days (Naegele's rule) gives Oct 12 as her EDD. At roughly 11 weeks today, she is still in the first trimester — prenatal vitamins and the NT scan (weeks 11–13) are the priorities now. The anatomy scan window opens in mid-May around week 18–20.
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Maya, 32 — Irregular 35-Day Cycle
How a longer cycle shifts the due date by a full week compared to the standard 28-day formula
Last Period (LMP)
Dec 9, 2024
Cycle Length
35 days
Adjusted Due Date (Parikh)
Sep 22, 2025 — Virgo ♍
Naegele (28-day assumed)
Sep 15, 2025
Parikh (cycle-adjusted)
Sep 22, 2025
Difference
+7 days
Ovulation (actual)
Dec 30, 2024 (Day 21)
If Maya used the standard Naegele formula, she'd get Sep 15 — but her 35-day cycle means ovulation happened on Day 21, not Day 14. Parikh's rule corrects for this, shifting the EDD to Sep 22. The 7-day difference matters clinically: a Sep 15 estimate could lead to an "overdue" diagnosis a week too early.
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Understanding Your Due Date
Due Dates Are Estimates
Only about 5% of babies arrive on their exact due date. The normal range for delivery is 37-42 weeks. Think of your due date as the middle of a 5-week window rather than a precise target.
Why Different Methods Give Different Dates
Naegele's Rule assumes all women have 28-day cycles and ovulate on day 14. Mittendorf-Williams accounts for the fact that first pregnancies tend to go longer. Parikh's formula adjusts for your actual cycle length. An early ultrasound measures the embryo directly, making it the most accurate single method.
First Trimester Ultrasound
An ultrasound between 8-12 weeks is the most accurate way to date a pregnancy, accurate to within 3-5 days. If it differs from the LMP date by more than 7 days, the ultrasound date is typically used.
Factors That Affect Timing
First pregnancies tend to go slightly past the due date. Subsequent pregnancies often come a few days earlier. Cycle length matters - longer cycles push the due date later. Genetic factors also play a role.