What is the best to approach a pregnant women about doing a belly bonding video similar to what preggohero does?

Take some portraits of Andrew Jackson along and You should be all good.

Just the same way you would approach any stranger. Say hello, talk about some kind of easy common topic like 'Damn, isn't it hot today?' or 'How are you doing?'

Showing a bit of consideration to her situation is always welcome if she's had a long day, so any way you can put that across is going to be positive.

Ask if she needs a hand with something. Be as NON-CREEPY as possible. Cannot stress this enough. SO many approaches I've received from guys (both before and during pregnancies) have come across as just plain creepy. If she seems uncomfortable, don't push conversation. Just keep it light-hearted and friendly, and if she feels a rapport with you, she'll smile and be friendly back. Then you could mention how fascinated you are by the idea of growing life and go from there.

Just be super chilled out and pleasant, don't stand too close and be prepared to give up easily if she seems uncomfortable (keeps trying to walk away/deliberately avoids eye contact/makes a phone call).

Not sure how much of the above is relevant to you, but I've met too many blokes who are genuinely nice and have good intentions, but come across too eager, which translates to creepy unfortunately.

Don't take knockbacks personally! Pregnant women often have very trying days, and may not want to have a conversation with anyone, let alone a stranger.