What this calculator does
A gestational age calculator answers a different question to a due-date calculator: instead of predicting a future due date, it reports how far along a pregnancy is right now, in completed weeks and days, counted from the first day of the last menstrual period. This calculator also has a second mode, corrected age, for tracking a premature baby's development against their due date rather than their actual birth date.
Both figures answer the question "how old, developmentally, is this pregnancy or this baby, right now", which is a genuinely different framing to estimating when a birth is expected. Corrected age in particular is specifically used to adjust a baby's age for how many weeks early they were born, since a baby born eight weeks premature is not developmentally the same age as a full-term baby of the same number of weeks since birth.
The formula
Current gestational age counts the number of days from the first day of the last period to the reference date, then converts that into completed weeks and remaining days. Corrected age works differently: it starts from the baby's actual age since birth (chronological age), then subtracts however many weeks early the baby was born relative to full term (40 weeks), so the corrected figure tracks age from the original due date instead of the birth date.
| Term | Meaning |
|---|---|
| Gestational age | How far along a pregnancy is, in completed weeks and days, counted from the first day of the last period. |
| Chronological age | A baby's age counted simply from their actual date of birth, with no adjustment. |
| Corrected age | A premature baby's age adjusted to count from their original due date rather than their actual birth date, used for developmental comparisons. |
The inputs explained
| Field | What to enter |
|---|---|
| What to calculate | Choose whether to track current gestational age during a pregnancy, or corrected age for a baby already born premature. |
| First day of last period (gestational-age mode) | The first day of the last menstrual period, used only in gestational-age mode. |
| Calculate as of (gestational-age mode) | The date to calculate gestational age as of. Defaults to today. |
| Baby's date of birth (corrected-age mode) | The baby's actual date of birth, used only in corrected-age mode. |
| Gestational age at birth, in weeks (corrected-age mode) (weeks) | How many weeks pregnant the birth happened at (gestational age at birth), used only in corrected-age mode. Full term is 40 weeks. |
When to use it
Tracking how far along a pregnancy is right now
Rather than only knowing a predicted due date, current gestational age in completed weeks and days is the figure most commonly asked about and quoted at antenatal appointments.
Adjusting a premature baby's developmental age
Health professionals and parents often track a premature baby's milestones against their corrected age rather than their actual age since birth, so development is compared fairly against a baby born at full term.
Understanding why a baby seems "behind" on a milestone chart
A baby born several weeks early can look behind a standard age-based milestone chart when measured from their birth date, when in fact they are on track once compared against their corrected age instead.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How current gestational age builds up over the course of a pregnancy
The first day of the last period set at a range of distances in the past, showing how gestational age accumulates.
| First day of last period | Gestational age | Trimester |
|---|---|---|
| 10 weeks ago | 10 weeks, 0 days | First trimester |
| 20 weeks ago | 20 weeks, 0 days | Second trimester |
| 28 weeks ago | 28 weeks, 0 days | Third trimester |
| 37 weeks ago | 37 weeks, 1 days | Third trimester |
How corrected age compares with chronological age at different degrees of prematurity
A baby whose actual birth date is one year in the past, shown at a range of gestational ages at birth.
| Gestational age at birth | Corrected age | Chronological age |
|---|---|---|
| 24 weeks | 36 weeks, 1 days (8.3 months, approx.) | 52 weeks, 1 days |
| 28 weeks | 40 weeks, 1 days (9.2 months, approx.) | 52 weeks, 1 days |
| 32 weeks | 44 weeks, 1 days (10.2 months, approx.) | 52 weeks, 1 days |
| 34 weeks | 46 weeks, 1 days (10.6 months, approx.) | 52 weeks, 1 days |
| 37 weeks | Born at or near full term, so no correction is needed: chronological age applies | 52 weeks, 1 days |
Questions
How is this different from a due-date calculator?
A due-date calculator is forward-looking: it predicts a future delivery date from the first day of the last period. This calculator instead reports the current status, either gestational age as of a chosen date during pregnancy, or a premature baby's corrected age after birth.
Until what age is corrected age normally used?
Corrected age is typically used for developmental tracking up to around two years old, after which the gap between corrected and chronological age is generally considered to matter less. A health professional can advise on when it stops being relevant for a specific baby.
What counts as premature for the corrected-age calculation?
Full term is defined as 40 weeks of gestation. This calculator applies a correction for any birth before 37 weeks; a baby born at 37 weeks or later is treated as needing no correction, since that already falls within the standard full-term range.
Is this calculator a substitute for medical advice?
No. It applies a standard formula to the dates and weeks entered, but pregnancy and premature-birth development should be tracked with a midwife, obstetrician or paediatrician, who can account for factors this calculator has no way to know about.
To estimate a future due date instead of current progress, see the pregnancy due date calculator or, for an IVF pregnancy, the IVF due date calculator.