Pregnancy Calculator
Estimate your due date, current gestational age, and days pregnant from your last menstrual period, with the days left until your due date. Optional cycle length and conception date support, all calculated locally.
How this calculator works: three ways to estimate a due date
Enter the first day of your last menstrual period (LMP). The tool applies Naegele's rule (LMP + 280 days ≈ 40 weeks) to give your due date, then converts that into your current gestational age, days pregnant and days remaining. If you know your conception date, switch on "use conception date" for a closer personal estimate — medical dating counts from the LMP, which runs about two weeks ahead of actual conception.
| Method | Formula | When it fits | Notes |
|---|---|---|---|
| Naegele's rule (tool default) | First day of LMP + 280 days (40 weeks) | Regular cycles, around 28 days | Most widely used; no conception date needed |
| Conception date | Conception date + 266 days (38 weeks) | Conception date known, e.g. fertility treatment | Closer to the individual timeline |
| First-trimester ultrasound | From crown-rump length (CRL) | Irregular cycles, uncertain LMP | Most accurate before 14 weeks; clinicians defer to it |
Worked example: LMP of 1 January 2026 plus 280 days gives a due date of 8 October 2026. If the conception date is known to be 15 April 2026, the due date becomes 6 January 2027 (conception + 266 days).
If your cycle is not 28 days
Naegele's rule assumes a 28-day cycle with ovulation on day 14. With a longer cycle, ovulation shifts later and applying 280 days outright understates gestational age. The practical correction: add (your average cycle length − 28) days to the due date.
| Average cycle | Ovulation (approx.) | Due-date adjustment | Example (LMP 1 Jan 2026) |
|---|---|---|---|
| 21 days | Day 7 | −7 days | 1 Oct 2026 |
| 25 days | Day 11 | −3 days | 5 Oct 2026 |
| 28 days (standard) | Day 14 | 0 (baseline) | 8 Oct 2026 |
| 30 days | Day 16 | +2 days | 10 Oct 2026 |
| 32 days | Day 18 | +4 days | 12 Oct 2026 |
| 35 days | Day 21 | +7 days | 15 Oct 2026 |
With highly irregular cycles (varying by more than a week) or PCOS, this correction also drifts — in those cases the first-trimester ultrasound is the reference standard.
Counting gestational age: why the weeks matter
Gestational age counts from the first day of your LMP, seven days per week, written as "X weeks + Y days". Prenatal visits follow a predictable cadence — every 4 weeks up to 28 weeks, every 2 weeks from 28 to 36 weeks, then weekly until delivery — and the key screening tests sit inside narrow windows:
| Gestational age | Visit cadence | Key tests and checks |
|---|---|---|
| 6-13+6 weeks | First visit | Dating scan, blood and urine tests, blood type, blood pressure and weight; confirm intrauterine pregnancy |
| 11-13+6 weeks | — | Nuchal translucency (NT) scan for chromosomal risk |
| 14-19+6 weeks | Every 4 weeks | Second-trimester serum screening / NIPT, fundal height, fetal heart rate |
| 18-22 weeks | Every 4 weeks | Fetal anatomy scan (the detailed structural survey) |
| 24-28 weeks | Every 4 weeks | 75 g oral glucose tolerance test (gestational diabetes screening) |
| 28-36 weeks | Every 2 weeks | Growth and amniotic fluid assessment, blood counts, position check |
| 37 weeks onwards | Weekly | Electronic fetal monitoring, ultrasound for fetal size, fluid, placental maturity, position |
Map your current gestational age onto this table and you can see which window you are in — and what your next visit is likely to cover.
How much weight should you gain?
Gestational weight gain is not "the more the better". The widely used reference (Institute of Medicine, 2009) sets ranges by pre-pregnancy BMI:
| Pre-pregnancy BMI | Category | Total gain | Weekly gain, 2nd-3rd trimester |
|---|---|---|---|
| Below 18.5 | Underweight | 12.5-18 kg | 0.44-0.58 kg |
| 18.5-24.9 | Normal weight | 11.5-16 kg | 0.35-0.50 kg |
| 25.0-29.9 | Overweight | 7-11.5 kg | 0.23-0.33 kg |
| 30.0 and above | Obese | 5-9 kg | 0.17-0.27 kg |
Two pacing notes: first-trimester gain should stay within 0.5-2 kg, and the weekly rate above should hold steady rather than arriving in sudden jumps. Work from your weight before pregnancy, not today's reading — the BMI Calculator will do the arithmetic.
Worked example: 1.65 m tall and 55 kg before pregnancy → BMI ≈ 20.2, normal weight → recommended total gain 11.5-16 kg at 0.35-0.50 kg per week through the second and third trimesters.
How much more energy do you actually need?
Pregnancy raises energy needs, but by less than most people assume. Against a pre-pregnancy baseline, the standard increments are +340 kcal/day in the second trimester and +452 kcal/day in the third — roughly one glass of milk plus an egg. There is no extra requirement in the first trimester.
| Stage | Change vs. pre-pregnancy | In practice |
|---|---|---|
| First trimester (1-13 weeks) | No additional calories needed | Focus on folate, avoid alcohol and smoking |
| Second trimester (14-27 weeks) | +340 kcal/day | One extra snack; prioritise protein, calcium, iron |
| Third trimester (28 weeks on) | +452 kcal/day | Iron and calcium needs peak; watch sodium and swelling |
To size those additions against your own metabolic rate, use the Calorie Calculator — but note that pregnancy is not a time to run a calorie deficit, and intake should not fall below your non-pregnant requirement.
Foetal development milestones by week
Translating weeks into what is developing makes the screening calendar easier to follow (individual babies vary; these are common milestones):
| Gestational age | Foetal development | What you may notice |
|---|---|---|
| 8 weeks | Major organs begin to form; the heart is already beating | Morning sickness often peaks; keep up folic acid |
| 12 weeks | Face and limbs formed; embryo becomes foetus | Uterus rises out of the pelvis; nausea often eases |
| 16 weeks | Bones continue to harden; sucking motions appear | Bump starts to show |
| 20 weeks | Hearing develops; most parents feel first movements | Anatomy scan window |
| 24 weeks | Lungs developing; a key threshold for premature survival | Glucose tolerance test window |
| 28 weeks | Eyes open; sleep-wake cycles emerge | Third trimester; visits move to every 2 weeks |
| 36 weeks | Position usually settles; body prepares for birth | Breathing eases, urinary frequency increases |
| 37 weeks | Full term; organ maturity is sufficient | Weekly visits plus fetal monitoring |
Frequently Asked Questions
My cycle is 35 days — is this tool's due date still right?The tool assumes a 28-day cycle. With a 35-day cycle, ovulation is roughly a week later, so add 7 days (LMP of 1 Jan 2026 shifts the due date from 8 October to 15 October). Safer still: tell your clinician your cycle length and let the first-trimester scan confirm the dates.
The scan says a different gestational age than the calculator. Which wins?Before 14 weeks, the ultrasound wins. A first-trimester scan dates the pregnancy from crown-rump length with an accuracy of roughly ±5 days, whereas LMP dating depends on cycle regularity and how well the date was remembered. If the two differ by more than a week, clinicians usually revise the due date to match the scan.
I have passed my due date and nothing is happening. Should I worry?Not immediately. The due date marks 40 weeks, and delivery anywhere from 37 to 42 weeks is normal. After 41 weeks, however, placental function can decline, so monitoring usually increases and induction may be discussed between 41 and 42 weeks. Follow your obstetrician's plan.
Why is gestational age counted from my period rather than conception?Because the exact conception date is rarely known, while the first day of the last period is a reliable, memorable anchor. Counting from the LMP puts gestational age about two weeks ahead of actual conception (ovulation typically falls near day 14), and this convention is used consistently in obstetric practice worldwide.
If I am planning a caesarean, is the date set by the due date?No. Planned caesareans are generally scheduled after 39 weeks, with the date decided by your obstetrician based on position, foetal growth, previous uterine surgery and your own health — delivering too early raises the risk of newborn respiratory complications.
What to use the result for
The most practical value of this calculator is anchoring your pregnancy to a timeline: knowing your gestational age tells you which screening window you are in, roughly when the next visit falls, and how to plan leave and preparation. If you are still trying to conceive, pair it with the Ovulation Calculator.
Remember that this is a statistical estimate, not a clinical judgement. With irregular cycles, PCOS, fertility treatment or twins, dating and the due date must be confirmed by your obstetrician using ultrasound and clinical assessment.