Lock the app

Settings → Privacy → Lock with Face ID — the switch is named after what your phone has: Touch ID, Optic ID, or Lock with passcode. With it on, the app asks every time it opens and every time you leave it. Switching the lock back off asks too, so somebody holding your unlocked phone cannot simply turn it off.

What else it does

• A cover in the app switcher. When you swipe up, the app's card shows a plain cover instead of your log, so a glance over your shoulder shows nothing.
• A cover while locked, so nothing flashes before Face ID succeeds.

If Face ID fails

Your device passcode works as the fallback, always. If your iPhone has no passcode set at all, the app says it cannot confirm it is you, and the lock cannot be used — there is nothing to fall back to.

Hiding sections

Separately from the lock, Settings → What you see → Sections, cards and counting hides parts of the day sheet you do not use: symptoms, mood and sleep, tags, medications, the note, labs. Trying to conceive has its own switch in What you see. The four tabs are always there, and the photo diary cannot be hidden. Hidden or not, the data is left alone.