I have a daughter. We wany to have a son. Tell me what precaution we should take

Unfortunately you get what you are given. There are no tried and true ways to conceive a boy or a girl.

Only way to assure the gender of your child is to adopt one... and even then, you never know.

... meaning, sorry. No medical way to choose.

Well you have sex and hope you are lucky, that is about it

50/50 chance
Either way love them 100%