Elisa,
This 'nurse while pregnant and you'll starve the baby' has been around since
at least our great-grandmothers' time.  My greatgrandmother told my uncle that
he was so skinny because she nursed my grandfather through her pregnancy with
him.  Now the same uncle is larger than grandpa.  This issue does come up
often with ob's.  They fail to understand that lactation is a very energy
efficient process, especially with an older baby.  One friend nursed 2
toddlers through her pregnancy and delivered a 9+ lb baby.  He had to be
evicted with the assistance of castor oil and blue cohosh tincture- she had a
previous 10 pounder with shoulder dystocia and did not want to risk out of a
home birth. I could go on and on with this anecdotal eveidence...
        If the concern is for protein nutrition (which is a concern in
pregnancy), the ob might be comfortable if the mom eats 100+ grams of protein
a day (a la Brewer diet), and possibly lets him draw blood periodically for
albumin levels.  Albumin levels in blood are a sensitive indicator of dietary
protein sufficiency in pregnancy.  There are good standards available, due to
albumin levels' diagnostic utility in pre-eclampsia/eclampsia.  Hope this
helps.  She might also like to contact LLL for information on nursing through
pregnancy and tandem nursing (nursing siblings who are not twins).
Let us know if the ob is at all receptive to monitoring mom or fetal growth.
Catherine Watson Genna, IBCLC