Frequently asked questions
These answers explain how the calculator works, what the dates mean and where its limits are. If your question is not covered here, you can reach us through the contact page.
What does the calculator do?
You enter your baby’s due date (or birth date) and an optional nickname. The calculator then shows a calendar covering roughly the first twenty months, marks the ten estimated fussy windows, and tells you whether today falls inside one of them. Each window has a short summary, common signs and care ideas.
Should I enter the due date or the birth date?
Use the due date if you know it. The windows are counted from the due date because that reflects how mature a baby’s nervous system is. If your baby was born close to the due date, the birth date gives almost the same result. If your baby was born several weeks early or late, entering the due date gives a more realistic estimate.
My baby was born prematurely. Does the calculator still work?
Yes, but enter the original due date rather than the birth date. A baby born six weeks early is likely to reach each window roughly six weeks later after birth than a baby born on time. Many health professionals use the same idea, called corrected or adjusted age, when they look at early milestones.
How accurate are the dates?
They are estimates. The windows come from the Wonder Weeks model of fussy periods in infancy. Parents often recognise these spells, but research does not agree on exact timing and every baby is different. Some babies show a clear change, some show very little, and many show it a few days earlier or later. Use the calendar as a prompt to watch for tiredness and overload, not as a prediction.
My baby is fussy outside a highlighted window. Is something wrong?
Not necessarily. Babies can be unsettled for many reasons, including hunger, tiredness, a cold, teething, vaccinations, travel or changes at home. If the fussiness comes with fever, breathing difficulty, vomiting, poor feeding, fewer wet diapers or a baby who is unusually floppy or hard to wake, contact a doctor whatever the calendar says.
Is a Wonder Weeks window a medical or developmental diagnosis?
No. The calculator and the guides on this site are general parenting information. They cannot tell you whether your baby is healthy or developing as expected. Regular check-ups with your doctor, nurse or health visitor are the right place for that.
What is the baby temperament check?
It is a set of 20 observation questions about everyday behaviour, such as activity level, reactions to new things and how easily a baby settles. The result describes tendencies and suggests care ideas. It is not a personality test or a screening tool, and the same baby may answer differently at different ages.
Where is my baby’s information stored?
The nickname and date you enter are kept in cookies in your own browser and in the address of your personal calendar page so that the calendar works when you come back. We do not ask for a real name. If you choose to create a home-screen icon from a photo, a small resized copy of that image is stored on our server. The privacy policy explains this in detail.
How do I share the calendar with a partner or family member?
Open the menu on your calendar page and choose the option to copy the link. Anyone who opens that link sees the same calendar. The link contains the nickname and date you entered, so only share it with people you trust.
How do I start over or change the date?
Open the menu on your calendar page and choose the reset option. This clears the saved nickname, date and icon from your browser and returns you to the start screen.
Can I add the calculator to my phone’s home screen?
Yes. The menu includes instructions for adding the page to your home screen on common phone browsers, and an option to use your own photo as the icon.
Is this site connected to The Wonder Weeks book or app?
No. Wonder Weeks Calculator is an independent site. It is not affiliated with or endorsed by the authors or publishers of The Wonder Weeks. The guide pages here are written independently and describe general infant development.
Is the calculator free?
Yes. The calculator, temperament check and guides are free to use without an account. Some content pages show advertising, which helps cover the cost of running the site.
For general reference only, not medical advice.