Hi. I'm in my second year only, and have two cows. How do you know when..

Your cow will only give birth after you give it a miracle potion, and you have to be able to milk her, then you know you can get the cow pregnant. Once you give the cow the miracle potion, when you talk to it, every day it will give a count down until it gives birth.

You have to give the cow a cow miricle potion

All you need to do is if your cows are healthy buy the miracle potion for cows from Barley that guy on the farm and giv it 2 the cow you want to give birth. Hope I helped.