Answer first
What this calculator tells you
Find how many weeks and days pregnant you are from the first day of your last period. Match the week to the milestones your clinician tracks, then defer to their dating if it differs. Formula: Gestational age = days since the first day of the last period; weeks = ⌊days ÷ 7⌋; due date = that first day + 280 days. At the worked-example inputs, the completed weeks is 20. Holding every other input steady, moving first day of last period from April 17, 2026 to May 15, 2026 moves the result from 18 to 22.
Transparent method
The formula
Match the week to the milestones your clinician tracks, then defer to their dating if it differs.
Worked example
Example inputs
How to interpret the result
Pregnancy weeks are counted from the first day of the last period, about two weeks before conception. So at the moment of conception you are already counted as two weeks pregnant. A period that started on May 1 makes late September week 20 plus a few days. The same count runs to a due date at 40 weeks, which is when a clinician expects, not promises, delivery.
At the worked-example inputs the completed weeks is 20. It rises with as of date and falls as first day of last period increases.
Gestational age counted from the last period is an estimate that assumes a regular cycle and an accurate date. A dating ultrasound can move it by days, and your clinician's dating is the one that counts for care and for the due date.
Before you rely on it
What to check
Confirm the date is the first day of bleeding, not the last. A wrong start date shifts every week and the due date with it.
The common error
Where people go wrong with pregnancy week calculator
Counting from conception. That gives a number two weeks lower than the one your care team uses, so milestones and appointments will not line up.
Sensitivity evidence
How first day of last period changes the completed weeks
Holding every other input at the worked-example value, moving first day of last period from April 17, 2026 to May 15, 2026 moves the completed weeks from 18 to 22: a spread of 4, or 20% of the worked-example result.
| First day of last period | Completed weeks | Extra days | Estimated due date |
|---|---|---|---|
| April 17, 2026 | 22 | 6 | January 22, 2027 |
| April 24, 2026 | 21 | 6 | January 29, 2027 |
| May 1, 2026worked example | 20 | 6 | February 5, 2027 |
| May 8, 2026 | 19 | 6 | February 12, 2027 |
| May 15, 2026 | 18 | 6 | February 19, 2027 |
Every input, tested
Which input moves the completed weeks most
Of the 2 inputs, first day of last period moves the completed weeks most (2 across the range tested) and as of date moves it least (2).
| Input | Tested from | To | Completed weeks at each end | Swing |
|---|---|---|---|---|
| First day of last period | April 24, 2026 | May 8, 2026 | 21 to 19 | 2 (10%) |
| As of date | September 17, 2026 | October 1, 2026 | 19 to 21 | 2 (10%) |
Two variables at once
Completed weeks by first day of last period and as of date
Across the grid the completed weeks runs from 16 to 24. Moving first day of last period from April 17, 2026 to May 15, 2026 shifts it by 4 at the middle column, and moving as of date from September 10, 2026 to October 8, 2026 shifts it by 4 at the middle row, so neither is the bigger lever here.
| First day of last period \ As of date | September 10, 2026 | September 24, 2026 | October 8, 2026 |
|---|---|---|---|
| April 17, 2026 | 20 | 22 | 24 |
| April 24, 2026 | 19 | 21 | 23 |
| May 1, 2026 | 18 | 20 | 22 |
| May 8, 2026 | 17 | 19 | 21 |
| May 15, 2026 | 16 | 18 | 20 |
The highlighted cell is the worked example: 20.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| First day of last period | May 1, 2026 | Enter the first day of last period used in this calculation. |
| As of date | September 24, 2026 | Defaults to today. |
| Completed weeks | 20 | |
| Extra days | 6 | |
| Estimated due date | February 5, 2027 | |
Inputs, definitions and assumptions
First day of last period
Enter the first day of last period used in this calculation. The prefilled worked-example value is May 1, 2026.
As of date
Defaults to today. The prefilled worked-example value is September 24, 2026.
How to use this calculator
- 1Verify the inputs. Gather first day of last period and as of date from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the completed weeks at 20. Store your own version of it as Scenario A.
- 3Test one change. Start with first day of last period, the input with the biggest effect here: moving first day of last period from April 24, 2026 to May 8, 2026 takes the completed weeks from 21 to 19, a swing of 10% of the worked-example figure.
- 4Check the boundary. Read the interpretation boundary above before acting on the result.
People also ask
Frequently asked questions
How do you calculate pregnancy week?
Gestational age = days since the first day of the last period; weeks = ⌊days ÷ 7⌋; due date = that first day + 280 days. At the worked-example inputs the completed weeks is 20.
What does the pregnancy week result mean?
Match the week to the milestones your clinician tracks, then defer to their dating if it differs. At the worked-example inputs the completed weeks is 20. It rises with as of date and falls as first day of last period increases.
How much does first day of last period change the completed weeks?
Holding every other input at the worked-example value, moving first day of last period from April 17, 2026 to May 15, 2026 moves the completed weeks from 18 to 22, a spread of 4.
What are the limits of this pregnancy week calculator?
Gestational age counted from the last period is an estimate that assumes a regular cycle and an accurate date. A dating ultrasound can move it by days, and your clinician's dating is the one that counts for care and for the due date. The tables on this page test first day of last period only from April 17, 2026 to May 15, 2026; a value outside that range is not tabulated here.
Which input moves the completed weeks most in the pregnancy week calculator?
Ranked by how far each moves the completed weeks across the range tested: first day of last period (2, 10%) and as of date (2, 10%).
How much does as of date matter in the pregnancy week calculator?
The worked example uses September 24, 2026. Holding every other input at its worked-example value, moving as of date from September 17, 2026 to October 1, 2026 takes the completed weeks from 19 to 21, a swing of 10% of the worked-example figure.
What is the difference between LDL, HDL and non-HDL cholesterol?
LDL carries cholesterol out to tissues and is the main target for lowering. HDL carries it back toward the liver. Non-HDL is total minus HDL, and it counts every particle that can build up in arteries.
Does my blood pressure change through the day?
Yes. It is usually lower in sleep, rises on waking and moves with activity, stress, caffeine and posture. A few calm readings at the same time of day give a truer picture than one.
How is an IVF due date different from a regular one?
The conception date is known, so it counts from the transfer instead of the last period. The embryo's age at transfer sets the offset, and your clinic's dating takes priority over any calculator.
Sources and evidence
Free Calculators Online is independent and is not affiliated with or endorsed by the source organizations. Educational estimates only.