The relation between MDMA and lactation is not clear. As far as I know, there are no published studies on this particular issue. Available data on amphetamine show that «In dosages prescribed for medical indications, some evidence indicates that amphetamine might not affect nursing infants adversely.». On the other hand, data from methamphetamine show that meth is secreted to breast milk and it should be avoided.

http://toxnet.nlm.nih.gov/cgi-bin/sis/search/f?./temp/~LJwPas:1

http://toxnet.nlm.nih.gov/cgi-bin/sis/search/f?./temp/~LJwPas:2

In my opinion, it is basic to avoid use that can be harmful to third persons. A person can asume risks for himself but not for others, even less a baby. So there would be three options:

1) Wait to finish lactation and then use MDMA or amphs

2) Interrupt natural lactation (use artificial milk) and use MDMA or amphs

3) Collect enough milk for 3-4 days (using a breast-pump), keep it in the fridge and use it during the following days to MDMA or amphetamines use.