There is a theoretical risk that milk thistle could have an estrogen-like effect, so people with hormone-sensitive conditions such as endometriosis, uterine fibroids or cancers of the breast, uterus and ovaries should avoid milk thistle, particularly the above ground parts of the plant.*
Theoretical, theoretically=speculation, in theory, hypothetical
It is up to you to decide if this plant would be best to avoid for your particular situation or work with a naturopath who can work one on one with you to determine if this plant is best for you.
I am also concerned that this About.com article has no studies sited or references.
The safety of milk thistle in pregnant or nursing women is unknown.*
In my experience Milk Thistle increases mother's milk and is safe to take while breastfeeding when a mother needs help increasing her milk supply.
*
http://altmedicine.about.com/cs/herbsvitaminsad/a/Milk_Thistle.htm