Pregnancy Due Date Calculator

About Pregnancy Due Date Calculator

How Does Naegele's Rule Work?

Naegele's rule, dating back to 1812, assumes a 28-day menstrual cycle with ovulation on day 14. The formula is simple:

EDD = LMP + 280 days (40 weeks)

Or equivalently: LMP + 1 year - 3 months + 7 days.

Worked example: LMP = January 15

1. January 15 + 280 days = October 22
2. Or: January 15 + 1 year = January 15 next year, - 3 months = October 15, + 7 days = October 22

If you know the conception date (e.g., through IVF or tracked ovulation), the calculation is: conception date + 266 days = EDD. Conception typically occurs about 14 days after the LMP in a 28-day cycle.

How Accurate Is the Due Date?

Only about 4-5% of babies are born on their exact due date. The due date is really the middle of a window:

Timeframe	Births Within This Window
On the exact due date	~4-5%
Within 1 week of due date	~50%
Within 2 weeks of due date	~80%
37-42 weeks (term range)	~95%

First pregnancies tend to go slightly longer than subsequent ones. A first-trimester ultrasound (weeks 8-12) is generally the most accurate method for dating a pregnancy, especially if your cycle length is irregular or uncertain.

Gestational Age vs Fetal Age

	Gestational Age	Fetal Age (Embryonic Age)
Counted from	First day of last menstrual period	Date of conception
Includes pre-conception?	Yes - about 2 weeks before conception	No - starts at fertilisation
Full term	40 weeks	38 weeks
Used by doctors?	Yes - this is the standard	Sometimes in research
Relationship	Gestational = fetal + ~2 weeks	Fetal = gestational - ~2 weeks

The Three Trimesters

Trimester	Weeks	Key Developments	Common Experiences
First	1-12	All major organs begin forming, heartbeat detectable ~week 6, limbs develop	Nausea, fatigue, breast tenderness
Second	13-27	Rapid growth, movements felt (~weeks 18-22), anatomy scan (~week 20), viable from ~week 24	Often feel best - nausea subsides, energy returns
Third	28-40	Lungs mature, brain development accelerates, baby gains most weight	Back pain, fatigue returns, Braxton Hicks contractions

Key Pregnancy Milestones

Week	Milestone
Week 4	Implantation, missed period, pregnancy test becomes positive
Week 6	Heartbeat detectable on ultrasound
Week 8-12	First prenatal visit and dating scan
Week 12	End of first trimester, miscarriage risk drops significantly
Week 18-22	Anatomy scan (mid-pregnancy ultrasound)
Week 24	Viability threshold - survival possible with intensive care
Week 28	Third trimester begins
Week 37	Early term - lungs typically mature
Week 39-40	Full term - optimal time for delivery
Week 42	Post-term - induction typically offered

Term Classifications

Classification	Gestational Weeks
Very preterm	Before 28 weeks
Moderate to late preterm	28 - 36 weeks
Early term	37 - 38 weeks
Full term	39 - 40 weeks
Late term	41 weeks
Post-term	42+ weeks

Adjusting for Cycle Length

The general adjustment is: EDD shift = (your cycle length - 28) days, because Naegele's rule assumes a 28-day cycle with ovulation on day 14. If you ovulate later, your due date shifts later by the same amount.

• Longer cycle (e.g., 35 days): Ovulation likely occurred around day 21 instead of day 14. Add 7 days to the calculated due date.
• Shorter cycle (e.g., 24 days): Ovulation likely occurred around day 10. Subtract 4 days from the calculated due date.

A first-trimester ultrasound is the most reliable way to confirm dating if your cycles are irregular. Per ACOG Committee Opinion 700 (reaffirmed 2022), if a first-trimester ultrasound EDD differs from the LMP-based EDD by more than 5 days, the ultrasound date takes precedence. In the second trimester, the threshold widens to 7-10 days.

When Do Babies Actually Arrive? US Statistics

Most babies arrive inside the 37-42 week term window, but the distribution is not symmetric around the due date. First-time mothers (nulliparous) average about 41 weeks and 1 day at delivery, while second and later pregnancies (multiparous) average about 40 weeks and 3 days - roughly five days earlier, according to a 2013 study in Human Reproduction (Jukic et al., NIEHS).

The US preterm birth rate was 10.41% in 2024 per CDC provisional data (unchanged from 2023), the highest level in 15 years. The UK equivalent is closer to 7.6% (ONS, 2023 data published 2024). About 27% of US births were induced in 2024, and post-term (42+ week) pregnancies are now rare because induction is typically offered at or before 41 weeks.

Delivery Timing	Share of US Births (2024)	Notes
Preterm (before 37 weeks)	10.4%	Higher risk of NICU admission, breathing support
Early term (37-38 weeks)	~26%	Elective delivery before 39 weeks is discouraged without medical reason
Full term (39-40 weeks)	~57%	Lowest neonatal adverse-outcome rate (ACOG)
Late term (41 weeks)	~6%	Monitoring increases, induction often offered
Post-term (42+ weeks)	<0.5%	Rare due to routine induction policies

Survival at the Edge of Viability

Viability is not a single sharp line. US NICU survival data for 2020-2022 (published in Pediatrics, October 2024) show that survival rises dramatically day by day across weeks 22 to 25:

Gestational Age	Survival With Active Treatment	Trend vs 2014
22 weeks	35.4% (up to 41% in 2023)	Up from 25.7% in 2014
23 weeks	52.8%	Improving year-on-year
24 weeks	71.1%	Traditional viability threshold
25 weeks	82.0%	High survival, lower long-term morbidity

Between 23 and 26 weeks, chance of survival increases by roughly 2-4% per day, which is why antenatal corticosteroids (betamethasone or dexamethasone) are given to mothers at risk of premature delivery - they accelerate fetal lung development within 24-48 hours.

What a Dating Scan Actually Measures

A first-trimester ultrasound estimates gestational age by measuring the crown-rump length (CRL) of the embryo, which is the distance from the top of the head to the bottom of the torso. Between 7 and 13 weeks CRL correlates very closely with gestational age, with an accuracy of plus or minus 3 to 5 days. After 14 weeks, biparietal diameter (BPD), head circumference, abdominal circumference, and femur length replace CRL, and dating accuracy falls to plus or minus 7-10 days in the second trimester and plus or minus 2-3 weeks in the third.

This is why the NHS offers the dating scan at 11-14 weeks in the UK, and why IVF pregnancies (where the exact transfer date is known) rarely need re-dating. If you want to count exact weeks between your LMP and your scan date, the Date Difference Calculator gives a day-accurate span.

Common Mistakes When Estimating a Due Date

• Using conception date as LMP: If you pick the "LMP" mode but enter what you think is the conception date, your EDD will be two weeks too late. Use the Conception Date mode instead.
• Assuming a 28-day cycle when yours is 32+: Naegele's rule hard-codes day-14 ovulation. Longer cycles push the real EDD later; your dating scan will usually catch this.
• Counting from implantation, not fertilisation: Implantation happens 6-12 days after fertilisation. Gestational age is counted from LMP, not from implantation or the positive test.
• Confusing "weeks pregnant" with "weeks completed": Saying "I am 12 weeks" typically means you are in your 13th week - you have completed 12 full weeks. Clinical notation uses the format 12+3 (12 weeks and 3 days).
• Assuming the EDD is the delivery date: Only 4-5% of babies arrive on the EDD. The Royal College of Obstetricians and Gynaecologists (RCOG, UK) and ACOG both describe the EDD as a mid-point of a roughly 5-week window.
• Re-calculating after every scan: ACOG guidance is that once a dating EDD is set in the first trimester, it should not be changed later even if later scans disagree, because later-scan error ranges are wider. Your "official" EDD is the one set at the first dating scan.
• Ignoring cycle length on fertility apps: Many period-tracker apps estimate a due date using your actual average cycle length rather than a flat 28 days. If your app and this calculator disagree by a few days, that is usually why. Your clinician's ultrasound-derived date is the one that matters clinically.

Gestational Age Notation and How Midwives Record It

Clinicians write gestational age in a weeks+days format, for example 32+4 meaning 32 completed weeks plus 4 extra days (equivalent to 32 and 4/7 weeks). A pregnancy reaches 40+0 on the EDD itself. The notation matters because decisions about induction, steroids for lung maturity, and when to discourage elective delivery all key off specific week-and-day boundaries, not vague "around 8 months" descriptions. ACOG's term-pregnancy definitions use the same format: early term runs 37+0 through 38+6, full term 39+0 through 40+6, late term 41+0 through 41+6, and post-term is 42+0 and beyond.

If you are further along than this calculator allows you to select (the LMP field is capped at today's date), that usually means the date you entered is in the future. Check that the month and year are correct - date pickers default to today and can be easy to misclick.

Related Calculators

During pregnancy, several other calculations come up regularly. The Pregnancy Weight Gain Calculator uses US Institute of Medicine (IOM/National Academies) guidelines based on pre-pregnancy BMI to show the recommended weekly gain range. The BMI Calculator is the input to that weight-gain range, and the Calorie Calculator helps estimate extra daily energy needs (about +340 kcal/day in the second trimester and +450 kcal/day in the third, per ACOG). For couples planning the timing of a baby shower, the Countdown Timer counts down to any future date.

All calculations run entirely in your browser. No personal data is sent to any server.

Sources

• ACOG Committee Opinion 579 - Definition of Term Pregnancy
• ACOG Committee Opinion 700 - Methods for Estimating the Due Date
• AAP Pediatrics (Oct 2024) - Survival of Infants Born at 22-25 Weeks (NICU 2020-2022)
• CDC NCHS - Births: Provisional Data for 2024
• Jukic et al., Human Reproduction (2013) - Length of Human Pregnancy and Variation
• NHS - 12-week Dating Scan
• Naegele's Rule and the Length of Pregnancy - A Review (PubMed)