Fact
Gestational age explained
Gestational age is conventionally counted from the first day of the last menstrual period, not from conception. Because ovulation usually happens about two weeks into that cycle, gestational age is normally about two weeks greater than the time since conception. This is why people are often told they are further along than they expected.
This page explains a dating convention. It does not calculate how long you have been pregnant, and it never will, because that requires clinical assessment.
The convention
Clinicians count a pregnancy from the first day of the last menstrual period. That day is the start of week zero.
This is a convention, chosen because the first day of a period is something a person can often identify, while the day of conception usually is not. It is not a claim that a pregnancy exists at that point.
Why the number surprises people
Ovulation usually happens roughly two weeks after the first day of a period. If conception occurs around ovulation, then by the time someone is "six weeks pregnant" by the clinical count, conception was about four weeks earlier.
So the two numbers below are describing the same pregnancy.
| Way of counting | What it measures | Typical relationship |
|---|---|---|
| Gestational age | Time from the first day of the last menstrual period | The number clinicians and legal rules normally use |
| Time since conception | Time from fertilisation | Usually about two weeks less than gestational age |
They are not interchangeable, and swapping one for the other changes the number by about two weeks. That matters when a limit is expressed in weeks, whether the limit is clinical or legal.
When the last period is not a reliable starting point
The convention assumes a fairly regular cycle. Several common situations break that assumption:
- Irregular cycles.
- Not knowing or not remembering the date of the last period.
- Bleeding early in pregnancy that was mistaken for a period.
- Recent use of hormonal contraception.
- Recent pregnancy, so no period has occurred yet.
In these cases the date of the last period is a poor guide, which is one reason dating is normally confirmed another way.
What ultrasound can and cannot settle
Ultrasound is the usual way of confirming how far along a pregnancy is. It is more accurate earlier in pregnancy than later, and it produces an estimate with a margin, not an exact day.
It can also show things that matter clinically for other reasons, including whether a pregnancy is in the uterus. That is relevant to the concerns described on the urgent help page.
An estimate from ultrasound may differ from the estimate based on the last period. When they differ, clinicians follow professional guidance on which to use.
Why this site will not calculate it for you
There is no calculator on this page and there will not be one.
To calculate gestational age this site would have to ask for the date of your last menstrual period. That is health information about a sensitive topic. This site does not collect it, does not want it, and does not have any legitimate use for it.
A calculator would also produce a number without any of the checks that make the number meaningful: whether the cycle is regular, whether the remembered date is right, and what an ultrasound shows. A number produced without those checks can be wrong by weeks, and someone might rely on it.
Where the number is used
- Clinically, because which methods apply depends on it.
- In drug labelling, because approved use of medicines is stated in terms of pregnancy duration.
- In law, because some legal rules are written in terms of weeks. Those rules are separate from medical facts, and they change. The law and policy section covers how to check the official record.
Terms that are used loosely
- Trimester. A rough division into three parts. Useful in conversation, imprecise for anything that depends on a specific week.
- Full term. Refers to the end of pregnancy, and has specific clinical definitions that are not what most people mean in conversation.
- Viability. Not a single fixed week. It depends on individual circumstances and on available neonatal care, and clinicians describe it as a range rather than a line. Presenting it as one universal number misrepresents the clinical picture.
- Late-term abortion. Not a clinical term at all. It does not correspond to any defined stage, which is why professional language guidance discourages it.