Clear numbers, checkable sources, honest limits

Why these calculators exist

Most pregnancy questions arrive between appointments: at night, on a phone, in the ten minutes before a nursery pickup. A search usually returns one confident number and almost no explanation — a due date, an ovulation day, a week count. The number might be right, but nothing on the page tells you how it was reached, which assumptions it rests on, or what would change it.

That is the gap we wanted to close. A calculator should take the details you already know — the first day of your last period, your usual cycle length, the date of a positive test — and turn them into something you can use in a real conversation: the arithmetic in plain English, the assumptions written down, the guidance it follows named openly, and a result you can keep or pass to someone else.

So the standard we hold each page to is simple. Clear means the explanation is part of the tool rather than an afterthought at the bottom. Checkable means you can compare our figure with the one in your antenatal notes and see exactly where any difference comes from. Shareable means a saved result card is easier to discuss at an appointment than a number you half remember.

What we are arguing against is the calculator that answers a complicated question with a bare digit and a confident tone. A date without its method is not information; it is a rumour with a typeface.

What we do, and what we deliberately do not do

A tool is defined as much by its limits as by its features. Here is where we draw the line.

If a question needs clinical judgement — whether a symptom is serious, whether a test result is normal, which treatment to choose — the honest answer is a person, not a web page. We would rather leave a topic alone than build something that looks like advice.

How a page gets published

The order matters. A formula is never written first and justified afterwards: the guidance comes first, and the code has to match it.

1. Choose a question worth answering We start from questions that come up repeatedly and have a published, reasonably stable method behind them. If the honest answer is "it depends on your clinician", we do not build a calculator for it.
2. Implement the formula from guidance, not memory The method is written against the approach described in current WHO, CDC, NHS or ACOG material, and the arithmetic lives in one place so it can be read, tested and changed after review.
3. Check the numbers by hand Sample inputs are recalculated independently with a calendar and a calculator. The awkward cases get the most attention: leap years, cycle lengths at the edges of the accepted range, dates that do not exist such as 30 February, and results landing on a month or year boundary.
4. Date the page Every page carries a visible "Last updated" date so you can judge whether it still reflects current guidance, rather than trusting a page that looks fresh but is not.
5. Log corrections instead of quietly editing When something is wrong, we fix it and record the change in the update log at the bottom of the affected page. Silent edits make a site look accurate and keep it unreliable.

Who works on Baby Tools

Baby Tools is a small independent team. We are not a hospital, a clinic or a medical practice, and the site has no accounts, no paywall and no advertising at the time of writing.

We use AI tools to help draft explanatory text and write code. That is a practical choice for a team this size, and it comes with a rule: AI is a drafting aid, never an authority. Every formula, every number and every source on a page is checked by a person against the published guidance before the page goes live, and the same person is accountable for it afterwards. Mistakes that slip through are ours, not a model's.

We also want to be explicit about what we do not have. There is no named medical reviewer, no clinical advisory board and no doctor signing off these pages, and we will not invent one to look more credible. Nothing on this site has been reviewed or approved by a clinician. If you need clinical judgement, your own midwife, obstetrician or paediatrician is the right source — which is exactly what every tool page tells you.

Corrections policy

Calculators get things wrong, and a page that hides its mistakes is worse than one that admits them. If a date looks off, a formula does not match the guidance we cite, or a sentence claims more than the evidence supports, please tell us. The most useful report includes the page, the inputs you used and the result you expected, because one reproducible error is faster to chase than a general complaint.

Every confirmed correction is written into the update log at the bottom of the page it affects, saying what was wrong and what it now says. Where a correction changes a result, we would rather change it visibly than edit silently. A page that no longer meets our own standard is taken down rather than left online with a warning label. We cannot read every message the moment it arrives, but a confirmed error is recorded, not buried.

Where to read more

The detail behind these promises lives on two other pages. The editorial policy sets out the source hierarchy, the calculation rules every tool follows, and how pages are reviewed or withdrawn. The privacy policy explains what happens to the dates you type, which is very little: they stay in your browser, because the calculations are performed there.

If you would rather start with a tool, the home page lists every calculator, beginning with the due date calculator.

Update log

• November 10, 2026 About page published alongside the editorial policy and privacy policy. These three pages carry a grouped footer that links every tool, guide and policy page.