Why do most women only share their pregnancy after the first trimester? Is it alright to share during the first month?

yes it is okay, as long as both you and your spouse are comfortable with sharing. some of my friends prefer not to share as the risk of a miscarriage is higher. also they did not want to let their family and in law's know. Sometimes in-laws can be quite a handful. then again, there is nothing wrong and it's perfectly okay to share.