Pregnancy and baby
Due dates, gestational age, the IOM weight-gain ranges and child growth percentiles — dated by ACOG conventions, bracketed by the term window, and sourced to the report or reference that published each range.
Everything here hangs off one date
The estimated due date is the reference every other figure in this wing is measured against. It is 280 days after the first day of the last menstrual period - the convention ACOG publishes in Committee Opinion No. 700, and the same convention historically known as Naegele's rule.
Gestational age is then simply the distance from that starting point, expressed in the clinical shorthand of completed weeks and days: 12w 3d. The trimester a date falls in, the 37-to-42-week term window, and the gestational week that indexes the weight-gain ranges all derive from the same count. Get the anchor right and everything downstream follows; get it wrong and everything downstream is wrong together.
Which is why this wing spends so much of its copy on how firm that anchor is. ACOG states the convention assumes a 28-day cycle with day-14 ovulation, reports that about half of women recall their last period accurately, and treats a first-trimester ultrasound as the most accurate way to establish gestational age. Where a scan disagrees with the arithmetic, the scan is the measurement.
The weight-gain ranges belong to the IOM
The 2009 Institute of Medicine report Weight Gain During Pregnancy: Reexamining the Guidelines publishes total gain ranges by pre-pregnancy BMI: 12.5 to 18 kg for a BMI below 18.5, 11.5 to 16 kg for 18.5 to 24.9, 7 to 11.5 kg for 25.0 to 29.9, and 5 to 9 kg at 30 and above. Those figures live in a versioned data pack with the report's URL and the date the row was last checked, not in the code.
The tool shows where a gain so far sits relative to the range for the gestational week entered, and stops there. The range belongs to the IOM; the reading belongs to a clinician. There is no scoring, no traffic-light colouring, and no red state anywhere near a body metric on this site - that is a deliberate design rule, not an oversight.
The report is also honest about its own limits, and so is the tool. The weekly rate ranges assume a stated first-trimester gain of 0.5 to 2 kg, and the 2009 report publishes no weekly rate for twin pregnancies at all - so for twins this site returns the published total range and no weekly trajectory, rather than inventing one.
Percentiles are positions, not grades
A percentile says where a measurement sits in a reference population: the 40th percentile means 40 percent of the reference children measured below that value. It is a position on a distribution, not a mark out of a hundred, and a child at the 20th percentile is not behind a child at the 60th.
The arithmetic behind it is the LMS method, published by Cole in 1990. Each age and sex has three parameters - a skewness L, a median M and a coefficient of variation S - which convert a measurement into a z-score and then into a percentile. The tool shows that working rather than hiding it behind a curve, because the L parameter is exactly what stops the distribution being assumed symmetric.
The mid-parental height calculation from Tanner 1970 gets the same treatment: a target height with its published range attached, presented as an expectation drawn from a mid-century reference population rather than a prediction about a particular child.
Which reference population a percentile is against
A percentile is meaningless without naming the group it is measured against, and there is more than one group. The WHO growth standards describe how children grow under recommended conditions and are generally used for the first two years; the CDC growth charts describe a reference sample of children in the United States and are generally used above that age. The same measurement can land on different percentiles depending only on which chart was used.
This site therefore names the chart alongside every percentile it prints, and keeps the underlying LMS tables in a versioned data pack with the source and the date each table was fetched. Those tables are marked as pending until they have been fetched and spot-checked against published median values - a percentile computed from an unverified table would look exactly as authoritative as one computed from a verified table, which is precisely why the distinction is machine-checked here rather than trusted.
None of the growth tools returns a judgement. They return a position, the chart it is a position on, and the arithmetic that got there. A single point on a growth chart is far less informative than the same child's trajectory across several, and reading either belongs with a paediatric clinician.
The height predictor makes the same point in a way that is hard to miss. Tanner's band around a mid-parental target is plus or minus 8.5 cm, corresponding to roughly the 3rd to 97th centile of expected adult height. That band is 17 cm wide - about seven inches - which is the single most important thing the tool has to say, and the reason it is rendered as a range rather than as a number.
In-depth guides
Calculators for this topic
A due date calculator adds 280 days to the first day of your last period, the convention ACOG publishes, and shifts it when your cycle is not 28 days long. It returns the estimated due date, gestational age, trimester dates and the 37 to 42 week window both ACOG and the NHS call term.
This applies the 2009 Institute of Medicine ranges: it works out your pre-pregnancy BMI, names the category the IOM puts that in, and shows the total gain range for the whole pregnancy plus the cumulative range by the week you enter. Twin ranges are included, and flagged as provisional by the report itself.
A growth percentile calculator turns a child's height or weight into a centile using the LMS method Cole published in 1990, with the reference tables the WHO and CDC publish by age and sex. Enter an age, a sex and a measurement to see where it falls, and what a centile does not mean.
A child height predictor applies Tanner's mid-parental method: add 13 centimetres to the mother's height for a boy, or subtract it from the father's for a girl, then average the two. The answer comes with Tanner's own band of plus or minus 8.5 centimetres, which spans the 3rd to 97th centile.
A conception date calculator subtracts 266 days from an estimated due date, or adds 14 days to the first day of a last period, following the convention ACOG publishes. It returns a conception window rather than a single day, because sperm survive for several days and ovulation timing varies between cycles.
Common questions
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