I can only offer personal experience:

Had four miscarriages (#4 was my son's twin) prior to my first child being born.  The first three were second trimester.
Had three miscarriages between b/w baby #1 and #2 while nursing #1.  All were in the first trimester - before 8 weeks. 
I took progesterone with all my successful pregnancies (the miscarriages b/w kids were so early I was just realizing I was pregnant when I lost them so I didn't get in to the doctor to get an Rx in time) even when initial blood work showed normal hormone levels.  I was nursing two children when having babies #3 and #4 and haven't had a miscarriage since baby #2 that I know of.  (Breastfeeding keeps my cycles away and I've only ever had one period before getting pregnant with the next baby.)

What I have heard is that if a mother's doctor is giving the okay for sex (orgasm, really), then nursing shouldn't be a problem.  She is producing more oxytocin having an orgasm than she is nursing her toddler.  I don't think there's much, if any, research to back that up.  I think most, if not all, information about the safety of breastfeeding during pregnancy regardless of a mother's hx with pregnancy loss is anecdotal.    I personally do not believe that nursing contributes to / causes miscarriage; if I did I would have weaned my babies upon finding out I was pregnant again, especially given I'm considered "high risk" due to my hx of pregnancy loss.  If mother is concerned about the nursing, she could reduce the frequency or length of time at the breast just for her own peace of mind or to be able to tell her HCP that she has "cut down on the nursing" if she's being pressured to wean.  Ultimately, though, if her body is going to reject the baby, it's going to happen whether she is nursing or not.  I would suggest she discuss progesterone supplements with her doctor.  What I did with baby #1 (because I was having treatments to help me get pregnant) and baby #2 was start having blood draws every 3 days after I ovulated (so I knew within 72 hours that I was pregnant) and I was put on progesterone immediately.  With baby #1 I had a few shots in the early weeks b/c I was carrying twins and my levels were low but with subsequent babies, it was a vaginal suppository.  I took them until about 14 weeks gestation.  I felt safer taking it even if blood draws showed good numbers b/c they can drop and sometimes by then it's too late.  The progesterone will not do her or the unborn baby any harm if she doesn't need it.  At 8 weeks, she'd only bra taking it for another 6 weeks but it might give her peace of mind unless her doctor is certain that her progesterone levels are fine.

Don't know if that helps at all.

Margo

"God illuminates us, then sends us into utter darkness to shine for Him"
Chuck Swindoll's version of an old Jewish saying of unknown (to me) origin

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