If having drunk a half glass of beer would affect my baby at this 4 month stage of pregancy?

I went to a party and i drink beer light by half a glass. I thought its okay. But i read in your answers that we are not allowed to drink alcohol. Would it affect my baby that i was able to drink beer during that time?

What is best diet for a 4 month pregnant like myself?

Thank you for all your answers .

I will appreciate it.