The 10% "rule" came from old pediatric standards (probably set in the days of
formula feeding?) and has been in both medical and nursing pediatric textbooks
for
at least 30 years.  Even Lawrence talks about 10% as being an acceptable level
of  weight loss.  I would say that most pediatricians and family physicians
received this information during their training  and keep it as the "standard"
for weight loss.

The first clinically-based challenge to this that I heard was from Marianne
Neifert,
back about 5 years ago or so.  She based her challenge on data from clients
seen at the breastfeeding clinic in Denver where she was the pediatrician.  As
I recall, the data showed that breastfed babies who lost 7% or more of their
birth weight in the first week of life were more likely to have breastfeeding
problems/failure than were those who loss less than 7%.  Neifert has written
articles and made many presentations based on these data and, I feel, has
helped us to develop a "red flag" approach to weight loss.

In our hospital, we flag babies who have lost 7% or more at discharge in order
to be sure that they receive early weight checks, home visits, or lactation
support, depending on the problem (if one has been identified) or on available
resources (such as home visit program eligibility, connection to community
agencies, etc.).
Of course, one cannot accurately predict who WILL have a feeding problem,
particularly when babies may be discharged at 24 hours of age, but I feel
comfortable using Neifert's data to support the need for early follow-up, if
only to reassure the mother that things are going well.

Jeanne M. Brotherton, RNC, IBCLC
Bellingham, Washington
The Northwest Corner of the USA