How due date calculators estimate your delivery date, the difference between conception date and last period methods, and what to expect from pregnancy weight gain guidelines.
The standard method for estimating a due date is called Naegele's Rule, named after the German obstetrician who developed it. It calculates an estimated due date as 280 days (40 weeks) from the first day of your last menstrual period, assuming a typical 28-day menstrual cycle.
This is the method most due date calculators and clinicians use as a starting estimate, since the first day of the last period is usually easier for most people to identify precisely than the actual date of conception.
Get an estimated due date based on your last menstrual period.
Open Due Date Calculator →If you know your specific conception date — for example, through IVF or precise ovulation tracking — a more direct calculation can be used: 266 days (38 weeks) from the date of conception. This avoids the assumption of a perfectly typical 28-day cycle that Naegele's Rule relies on, which can make it more accurate for people with longer or shorter natural cycles.
| Method | Calculation | Best for |
|---|---|---|
| Last menstrual period (Naegele's Rule) | 280 days from LMP | Most pregnancies — standard clinical method |
| Conception date | 266 days from conception | Known conception date (IVF, precise ovulation tracking) |
A more precise estimate if you know your specific conception date.
Open Conception Calculator →It's worth setting realistic expectations: a due date calculator produces an estimate, not a guarantee. Only a small minority of babies are actually born on their precisely calculated due date. Most healthy pregnancies deliver somewhere within roughly a 1-2 week window either side of the estimate — this is entirely normal and not a cause for concern on its own.
Recommended pregnancy weight gain depends on your pre-pregnancy BMI category — there isn't one single target that applies to everyone:
| Pre-pregnancy BMI | Recommended Total Gain |
|---|---|
| Underweight (under 18.5) | Generally higher — around 12.5-18kg |
| Healthy weight (18.5-24.9) | Generally around 11.5-16kg |
| Overweight (25-29.9) | Generally lower — around 7-11.5kg |
| Obese (30+) | Generally lowest — around 5-9kg |
See how your weight gain compares to general guidelines for your pre-pregnancy BMI.
Open Pregnancy Weight Gain Calculator →Naegele's Rule, which underpins most due date calculators, assumes a twenty-eight day cycle with ovulation on day fourteen. Both assumptions are population averages rather than individual facts.
Cycle length varies considerably between people and between cycles for the same person. Because most of that variation occurs before ovulation rather than after it, a longer cycle usually means conception happened later than day fourteen — which shifts the true due date later than the formula suggests.
Research consistently finds that only a small minority of babies arrive on their estimated due date. The great majority of births occur across a window of weeks around it. The date is best understood as the centre of a range, not an appointment.
An early ultrasound measures the embryo or fetus directly rather than inferring gestational age from your cycle. Because early growth is relatively consistent between pregnancies, a first-trimester ultrasound generally provides a more reliable estimate of gestational age than a last-menstrual-period calculation.
This is why, where the ultrasound date and the calculated date differ by more than a defined margin, clinicians commonly adopt the ultrasound date. The reliability of dating by ultrasound decreases as pregnancy progresses, which is why an early scan carries more weight than a later one for this purpose.
If you have irregular cycles, were recently using hormonal contraception, are unsure of your last period, or conceived while breastfeeding, cycle-based dating is particularly unreliable and the scan matters more.
Calculators that estimate a conception date work backwards from the same assumptions and inherit the same uncertainty.
Sperm can survive for several days in the reproductive tract while the egg remains viable for roughly a day. This means intercourse on a particular day and conception on that same day are not the same event, and the interval between them is unknown. A conception date derived from a calculator identifies an approximate window rather than a day.
Recommended weight gain during pregnancy varies according to pre-pregnancy body mass index and whether you are carrying more than one baby. The ranges published by health authorities are guidance for clinicians and expectant parents, and they are ranges rather than targets.
Gaining outside the recommended range is associated with some increased risks, and this is a reason to discuss it with your midwife or doctor rather than to attempt to correct it independently. Deliberately restricting food intake during pregnancy is not advised and should never be undertaken without medical supervision.
Contact your midwife, GP, or hospital promptly if you experience bleeding, severe or persistent abdominal pain, severe headache or visual disturbance, sudden swelling of the face or hands, reduced or changed fetal movements, fever, or fluid loss. These are not exhaustive, and if something feels wrong it is always appropriate to seek advice.
If you are pregnant or think you may be, book with a GP or midwife early. Early pregnancy care, including a dating scan, meaningfully improves the accuracy of everything discussed on this page.
Pregnancy is conventionally described in weeks of gestation counted from the first day of the last menstrual period. Because ovulation typically occurs around two weeks into that count, you are considered two weeks pregnant at the moment of conception — a convention that reasonably confuses almost everyone the first time they encounter it.
Clinicians describe a range of weeks around the due date as term, rather than treating the estimated date as a boundary. Births before and after that range are managed differently, which is one reason accurate dating matters clinically even though the specific day is rarely accurate.
Your maternity care provider will discuss what happens if pregnancy continues past the estimated date, and the approach varies according to individual circumstances. This is a conversation to have with them rather than a decision to reach from a calculator.
Several common situations make last-menstrual-period dating unreliable: irregular cycles, recent use of hormonal contraception, conception while breastfeeding, uncertainty about the date of the last period, and pregnancies conceived through assisted reproduction where the timing of transfer or insemination is precisely known and should be used instead.
In each of these cases an early ultrasound provides substantially better information than any calculator can. If you fall into one of these groups, treat an online estimate as a placeholder and arrange early pregnancy care promptly.
A due date calculator applies a formula built on average cycles to your last menstrual period. It is a reasonable starting estimate and an unreliable prediction of the day of birth. An early ultrasound dates a pregnancy more accurately, and where the two differ, clinicians generally rely on the scan.
Conception dates carry the same uncertainty in reverse, and pregnancy weight gain figures are population guidance rather than individual targets.
This page provides general information only and is not medical advice. Your midwife, GP, or obstetrician should be your source of truth for dating, weight, and every clinical question during pregnancy.