Where to place the first message in a chat?

Imagine a simple messenger app. Where to place the first message(s) in a chat? See examples below.

A or B in a messenger

E.g. Slack, Facebook's Messenger does it the A way. Snapchat, WhatsApp does it the B way.

I'd argue the B way is more correct due to F reading pattern, however, can someone shed me some light on this?