There's not much point in a mammal producing milk while its pregnant. Dairy cows do though. Dairy farmers aim for a cow giving birth every 365 days. After she has a calf, most farmers allow her one ovulation and try and impregnate her on the second or third ovulation, 6 - 9 weeks after giving birth. She lactates while pregnant for seven months, producing on average around 8,000 litres of milk.