A natural Estrogen that is marketed for oral, injection and topical administration (skin and vaginal).On the chemical form of valerate, it is used in association with a progestin as a combined birth-control compound (e.g. Estradiol + Dienogest) Estradiol is excreted into breast milk in clinically non-significant amount (Nilson 1978) and no problems have been observed in infants whose mothers were treated (Pinheiro 2016). Plasma levels of these infants were undetectable or very low (Pinheiro 2016). After administration in the form of transdermal patches milk levels have been undetectable (Pinheiro 2016, Perheentupa 2004). Despite these data, an older publication associated the use of transdermal estradiol with a case of jaundice and poor weight gain (Ball 1999).There is greater passage to milk when the administration is vaginal. There is evidence (albeit inconsistent) that estrogen-containing pills may decrease milk production, especially during the first few weeks postpartumThey may reduce the protein content of the milk. No problems have been observed in infants whose mothers were treated, except some cases of transient gynecomastia in infants whose mothers were receiving a higher dose than usual. Estrogen exposure in childhood or adolescence, does not influence the subsequent production of milk. The American Academy of Pediatrics states that this medication is usually compatible with breastfeeding.