View Larger Version | Estrogen levels vary by type of milk, but when a cow is milked also affects the content. U.S. cows are milked 10 months a year (light blue) and are pregnant for nine (darker blue). Estrone and estradiol levels increase in the cow (Plasma E1/E2) and her milk (Milk E1/E2) during pregnancy. The cows are dry, or not milked (yellow), for two months before giving birth, when estrogen levels are highest. One study has shown that estrogen levels are lower in milk from cows kept by a nomadic group in Mongolia that milks cows for human drinking just six months a year and only early in pregnancy. Credit: Sources: Graph from Zambito et al./Journal of Dairy Science; Calendar adapted from Akio Sato and Ganmaa Davasaambuu