Your due date is estimated by adding 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your cycle length. Only about 4–5% of babies arrive exactly on that date — most come within two weeks either side. This free calculator computes your due date and current week; a first-trimester ultrasound finalizes it.
“When is my baby due?” is the first calculation of every pregnancy — and the answer is a 280-day rule with a two-century pedigree. This pregnancy due date calculator applies it (with the cycle-length correction most tools skip), shows how far along you are today, and then explains what a due date honestly is: a well-centred estimate, not an appointment.
Get your due date below, then read on: how the calculation works, how many weeks pregnancy really lasts, how accurate due dates are — and when doctors officially change them.
Pregnancy Due Date
Estimated due date from last period (Naegele rule).
How is a due date calculated?
The standard method — Naegele’s rule — adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). It counts from the period rather than conception because the period date is usually known precisely, while conception rarely is. Two refinements matter:
- Cycle length correction: the 280-day rule assumes a 28-day cycle with ovulation on day 14. Longer or shorter cycles shift ovulation — so the calculator adds or subtracts the difference (a 32-day cycle pushes the due date 4 days later).
- Known conception or IVF dates: when the fertilization day is actually known, counting 266 days from conception — or 261/263 days from a day-5/day-3 embryo transfer — is more direct; our conception calculator handles that direction.
How many weeks is a pregnancy?
Forty weeks by the LMP clock — but “term” is a window, not a point, and modern obstetrics names its parts:
| Weeks | Classification |
|---|---|
| Before 37+0 | Preterm |
| 37+0 – 38+6 | Early term |
| 39+0 – 40+6 | Full term — the target zone |
| 41+0 – 41+6 | Late term |
| 42+0 and beyond | Post-term — monitored closely, often induced |
Week-by-week tracking — trimesters, scan windows, milestones — is the job of our gestational age calculator, which answers “how far along am I?” in detail.
How accurate is a due date?
Honestly: only about 4–5% of babies arrive on their due date. The estimate is well centred — most births cluster within two weeks either side — but spontaneous labour has its own schedule: roughly two-thirds of babies arrive within a week of the date, first pregnancies tend to run slightly longer, and a due date is best read as “due month, centred here”. That is not a flaw in the math; it is the biology the math summarizes.
Can my due date change?
Yes — and there is an official rule for it. A first-trimester ultrasound measures the embryo’s crown-rump length and dates the pregnancy to about ±5–7 days, which beats period math whenever cycles are irregular or the LMP date is soft. If the early scan differs from the LMP-based date by more than about 5–7 days, the scan date becomes the official due date — once, early, and rarely again afterwards (later scans measure growth, not age, and are poor for dating). One early accurate date matters clinically: screening windows, growth assessment and decisions about induction all hang on it.
From positive test to due date: the first steps
A practical sequence for the first weeks: confirm with a home test from the missed period; book the first antenatal visit (typically weeks 6–9) where dating is confirmed by ultrasound; begin folic acid if you haven’t already; and review medication and supplements with your physician. Bleeding with pain, severe one-sided pain, or a positive test with a history of ectopic pregnancy deserve prompt medical review rather than waiting for the scheduled scan. Acibadem’s obstetrics team — including the perinatology unit for high-risk pregnancies — covers the journey from dating scan to delivery.
This tool is informational. Your official due date is set by your obstetrician, usually from a first-trimester ultrasound — always follow your own care team’s dates and appointment schedule.
Pregnancy week by week: the milestone map
Between the positive test and the due date lies a fixed calendar of appointments and milestones worth knowing in advance:
| Weeks | Milestone |
|---|---|
| 6–9 | First visit and dating scan; heartbeat visible from ~6–7 weeks |
| 11–14 | First-trimester combined screening; NIPT blood testing possible from week 10 |
| 16–20 | Movements first felt (later in first pregnancies) |
| 18–22 | Detailed anatomy scan — the long, thorough ultrasound |
| 24–28 | Glucose screening for gestational diabetes |
| 28+ | Third trimester: growth checks, Rhesus management where relevant, kick-count awareness |
| 35–37 | Group B strep screening (practice-dependent); birth plan finalized |
| 39–41 | Full term — the target window; monitoring intensifies past 41 |
Every one of these windows is calculated from the due date — which is why an accurate early date matters more than any later measurement.
Due dates for twins and multiples
The calculator’s 40-week math assumes one baby. Twins are dated the same way, but delivery expectations differ: about half of twins arrive before 37 weeks, and most guidelines plan delivery at 37–38 weeks for uncomplicated twin pregnancies (earlier for twins sharing a placenta) rather than waiting for the singleton due date. Practically: with multiples, treat the calculated due date as an outer boundary you are unlikely to meet, and let your obstetric team’s schedule lead — multiple pregnancies are routinely managed with closer monitoring from the start.
Signs labour is approaching
Near the due date, the body usually announces itself in stages: the bump “drops” as the baby engages (lightening), practice contractions get more noticeable, and the mucus plug may pass — a one-off sign that can still precede labour by days. The distinction that matters is Braxton Hicks vs true labour: practice contractions are irregular, don’t intensify, and fade with rest or a change of position; true contractions become regular, longer, stronger and closer together regardless of what you do. Go in — or call — for regular contractions ~5 minutes apart, waters breaking (note the time and colour), bleeding, or reduced baby movements at any point. Reduced movements are always a same-day call, never a wait-and-see.
If your due date passes: induction, honestly
Reaching 40 weeks with no baby is common and, by itself, fine — monitoring simply steps up. Most guidelines recommend discussing induction by 41 weeks, because beyond 42 the placenta’s reserve declines and stillbirth risk, while still small, rises measurably. Induction methods (membrane sweep, prostaglandins, balloon catheter, oxytocin) are chosen by cervical readiness, and a large trial base shows induction at 41 weeks does not raise caesarean rates versus waiting. The decision is individual — baby’s size, your health, cervical status — and belongs in a conversation with your obstetrician, ideally had calmly at 40 weeks rather than urgently at 42.
What actually starts labour — and the folklore that doesn’t
The trigger is a hormonal conversation between a mature baby, the placenta and the uterus — which is why no due-date hack reliably starts it. The folklore, tested: spicy food, bumpy car rides, pineapple, raspberry-leaf tea, full moons — none survive controlled study (maternity wards are no busier at full moon, however firmly staff believe otherwise). Two methods have genuine partial evidence at term: nipple stimulation (releases real oxytocin — effective enough that it should only be tried after discussing with your midwife or obstetrician) and sex at term (plausible mechanisms, mixed trial results, safe in uncomplicated pregnancies). Walking won’t start labour but helps once it has. The honest summary: a healthy pregnancy at 40 weeks needs patience or a medical induction decision — not pineapple; and anything promising to “naturally induce” before term is working against the baby’s interests, not for them.
Practical countdown: what to have ready by when
A due date is also a project deadline. A sane sequence: car seat sourced and fitted by ~36 weeks (hospitals check); hospital bag packed by 36–37 (documents, chargers, clothes for both of you, going-home outfit); birth preferences discussed with your team by 37; the route to the hospital — and who drives at 3 a.m. — agreed the same week. After 37 weeks, keep the phone charged and the tank full: “full term” means the countdown is live. Acibadem’s obstetrics team walks internationals through the full logistics, from dating scan to discharge.
Frequently asked questions
How do I calculate my due date?
Add 280 days (40 weeks) to the first day of your last period, adjusting by the days your cycle differs from 28. From a known conception date, add 266 days instead.
What percentage of babies are born on their due date?
About 4–5%. Roughly two-thirds arrive within a week of the date, and the practical planning unit is a two-week window either side.
Why does the doctor’s due date differ from my calculation?
Usually because a first-trimester ultrasound re-dated the pregnancy — scan dating overrides period math when they differ by more than about 5–7 days, or when cycles are irregular.
How many weeks pregnant am I?
Count the weeks since the first day of your last period — the calculator shows it alongside your due date, and our gestational age calculator breaks it into trimester and milestones.
Does a due date from IVF work differently?
Yes — it is more precise, because the fertilization day is documented: transfer date + 261 days (day-5 embryo) or + 263 days (day-3). No estimation involved.
What happens if I go past my due date?
Nothing alarming at first — late term (41 weeks) is common, monitored with extra checks. Most guidelines discuss induction by 41–42 weeks, individualized by your obstetrician.
Do first babies usually come late?
Slightly — first pregnancies average a few days longer, and are less likely to deliver before the due date. It is a statistical lean, not a rule worth planning around.
Is a due date from conception more accurate than from my period?
Only if the conception date is genuinely known (IVF, single tracked ovulation). Otherwise period-plus-ultrasound dating is the standard — and the early scan settles disagreements either way.
Can my due date change twice?
It shouldn’t — dating is fixed by the earliest reliable ultrasound and then left alone. Later scans showing a “bigger” or “smaller” baby describe growth, not a new age, and don’t move the date.
Is it safe to deliver at 36 or 37 weeks?
Babies born at 37–38 weeks (early term) generally do well but have slightly higher rates of breathing and feeding wobbles than 39+ — which is why planned deliveries without medical reason are scheduled from 39 weeks. Before 37 is preterm and needs neonatal-aware care.
How is a twin due date different?
The 40-week math is the same, but delivery is usually planned at 37–38 weeks for uncomplicated twins — treat the calculated date as an outer boundary, not the plan.
Numbers are a starting point — not a diagnosis
If your results raise questions, Acibadem specialists can review your situation properly: share your case and receive a free, no-obligation medical opinion from the relevant department.