Due Date Calculator
Enter the first day of your last period — or your conception date if you know it — and get your estimated due date, trimester boundaries, and a week-by-week timeline. Everything runs in your browser.
All date calculations happen locally in your browser. We never store or track your personal health data.
What this calculator does
It answers the question every newly pregnant person has within the first few hours: when? Using the clinical standard known as Naegele's Rule, the calculator takes the first day of your last menstrual period (LMP) and adds 280 days — 40 weeks — to produce your estimated due date (EDD). If you tracked ovulation or conceived through IVF and know your conception date precisely, it can calculate from that point instead (adding 266 days, or 38 weeks).
Beyond a single date, the tool also maps out your three trimester boundaries and flags clinically meaningful milestones — the kinds of markers your midwife or OB will reference at each appointment.
First-time parents
You've just seen two lines on a test and want to know how far along you are before your first appointment. This gives you a working estimate to share with your provider.
People doing IVF or fertility tracking
When you know your exact ovulation or retrieval date, the conception-date method gives a more precise result than LMP alone.
Anyone double-checking
Your doctor gave you a due date from an early ultrasound. You want to cross-reference it or understand how it relates to your cycle.
How the calculation works
Method 1 — Last menstrual period (LMP)
Naegele's Rule has been the clinical standard since the early 1800s and remains the default because most people know the approximate date of their last period, even if they don't know exactly when they ovulated.
The formula
LMP + 280 days = EDD
(Equivalently: LMP + 1 year − 3 months + 7 days)
The 40-week figure includes the roughly two weeks before ovulation in a standard 28-day cycle. This is why pregnancy is counted from the LMP rather than conception — fertilisation typically happens around day 14, not day 1.
Method 2 — Conception date
If you know the day of conception — through IVF, precise ovulation tracking, or basal body temperature charting — the formula is simpler and often more accurate.
The formula
Conception date + 266 days = EDD
(38 weeks from fertilisation)
This method bypasses the assumption of a 28-day cycle entirely, which makes it more reliable for people with irregular periods or long/short cycles.
Reading your result
Your EDD is a midpoint, not a deadline. Here's what the numbers around it actually mean clinically:
Preterm
Before 37 weeks
Delivery before this point requires specialist care. Your provider will have been monitoring closely.
Early term
37–38 weeks
Baby is mostly ready but benefits from a little more time. Elective induction isn't typically offered yet.
Full term
39–40 weeks
The target window. Most of the developmental fine-tuning — brain, lungs, liver — finishes here.
Late term
41 weeks
Many healthy babies arrive here. Your provider may begin additional monitoring of the placenta and fluid.
Post-term
42+ weeks
Most providers recommend induction at or before this point due to placental aging and increased risk.
Trimester milestones at a glance
First trimester
Weeks 1–13
- ›Heart begins beating around week 6
- ›Major organs form (organogenesis)
- ›First prenatal visit + blood panel
- ›Nuchal translucency scan (weeks 11–13)
- ›Miscarriage risk drops sharply after week 12
Second trimester
Weeks 14–27
- ›Anatomy scan (weeks 18–22)
- ›Quickening — first fetal movements
- ›Gender may be visible via ultrasound
- ›Glucose screening (around week 24–28)
- ›Viability threshold around week 22–24
Third trimester
Weeks 28–40+
- ›Lung maturation — critical final stretch
- ›Baby turns head-down (36 weeks ideally)
- ›Group B Strep test (35–37 weeks)
- ›Weekly appointments from week 36
- ›EDD arrives — labor may begin any time
Practical tips for using your EDD
Plan for a window of two to three weeks on either side of your EDD. Hospitals and birth centers see entirely normal deliveries from week 37 to week 42. Mentally anchoring too hard to a single date can increase anxiety without improving outcomes.
Some people prefer to share a rough month rather than a specific date, to avoid pressure from well-meaning friends and family as the date approaches. That's a completely reasonable approach.
If an early scan places your EDD more than 5–7 days from the LMP-based estimate, your provider will typically use the ultrasound date going forward. That's normal — early fetal size is the most reliable measure we have.
Knowing your second trimester starts around a specific date lets you plan ahead — scheduling the anatomy scan, booking birthing classes, or timing maternity leave conversations with an employer while you still have runway.
If your cycles are routinely 35–40 days rather than 28, your actual ovulation — and therefore conception — happened later than Naegele's formula assumes. An early ultrasound will correct for this. Don't panic if your scan date is a week later than this calculator shows.
The EDD calculation is identical for twins and triplets. But multiple pregnancies are routinely delivered earlier — around 37–38 weeks for twins — so your provider's planning will diverge from the 40-week milestone well before your EDD arrives.
Limitations to keep in mind
This tool is built on the same formula your OB uses — but it works with the information you give it, not an ultrasound. A few things it can't account for:
- !Cycle length variations — if your periods aren't 28 days, the LMP method may be off by a week or more.
- !Late or missed ovulation within a cycle, which delays conception without changing the period date.
- !Embryo development rate differences, which an early scan can detect but a date calculator cannot.
- !IVF-specific nuances like frozen embryo transfer age, blastocyst vs. day-3 transfer timing.
- !Medical conditions (PCOS, thyroid issues) that affect cycle regularity and hormone-based dating.
This calculator is a starting point — a useful, clinically grounded estimate. Your healthcare provider will confirm and refine it at your first appointment.
Your pregnancy data stays yours
Many popular "free" pregnancy apps monetise by selling behavioural and health data to advertisers and data brokers. Pregnancy is a particularly sensitive category — the data you enter during those 40 weeks can follow you for years.
This tool is different by design. The calculation happens entirely in your browser using local JavaScript. Nothing is sent to a server. There's no account, no tracking pixel, and nothing stored when you close the tab. There's simply nothing to sell.
Common questions
How accurate is an estimated due date?
Statistically, only about 4–5% of babies are born on their exact due date. Most arrive within 10 days of the estimate in either direction. The EDD is a statistical midpoint based on population averages — useful for scheduling and monitoring, but not a precise prediction for any individual pregnancy.
What's the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period, while fetal age (or embryonic age) is counted from conception — usually about two weeks later. When your doctor says you're 10 weeks pregnant, they almost always mean gestational age. This calculator uses gestational age, consistent with how pregnancy weeks are counted clinically.
Can I still use this if my cycle isn't 28 days?
Yes, though the LMP method will be less precise. If your cycle is consistently longer than 28 days, your EDD from this calculator may be slightly earlier than reality. For example, if you have a 35-day cycle, ovulation typically happens around day 21 rather than day 14, meaning conception — and your true EDD — is about a week later. Your first-trimester ultrasound will establish a more accurate date.
My IVF clinic gave me a different date. Which is correct?
Your clinic's date. IVF pregnancies are dated precisely from the retrieval or transfer date, and clinics adjust for blastocyst age (a day-5 embryo is dated 5 days ahead of fertilisation). Use your clinic's EDD — it's based on direct knowledge of when your embryo was fertilised.
Is a due date the same as an induction date?
No. Your EDD is when a full-term pregnancy would naturally conclude on average. Induction decisions are based on clinical assessment — placental function, fluid levels, fetal movement, and your individual history. Most providers won't discuss induction until at least 39 weeks, and many healthy pregnancies continue until week 41 or 42 without intervention.
What does 'full term' mean, and why does it matter?
Full term is 39–40 weeks. Research shows that babies born in this window — even just a few days before — have better outcomes in terms of lung maturity, feeding ability, and brain development than those born at 37–38 weeks (early term). This is why routine elective inductions before 39 weeks are generally avoided unless there's a medical reason.