The term 'not thriving' simply means a baby isn't growing as he should.
Each baby has to be assessed as an individual, but for practical purposes something may be wrong if a baby continues to lose weight after 10 days of life, hasn't regained his birth weight by three weeks, gains at a very slow rate after the first month, or gains at an increasingly slow rate. There’s particular reason for concern if he is also unhealthy.
There are many reasons why a breastfed baby may not grow as he should. He may not drink enough milk - most likely because of poor positioning at the breast, infrequent short feeds, or poor sucking or swallowing. Mum may currently not be producing enough milk because of poor technique, an unhealthy diet, illness or exhaustion. Or she may not be letting her milk down well - because her baby isn’t positioned well, or for psychosocial reasons or because she's a heavy smoker or drinker or takes certain drugs. An unreliable let-down reflex eventually leads to poor milk production. It’s vital to check whether you need to correct poor positioning, improve your breastfeeding technique, and increase your milk supply.
The baby may be losing nutrients because of malabsorption, diarrhoea or vomiting; may have an infection (for example in his urine); or may have high energy needs (for example, a baby who is small for dates or who has a congenital heart disease) which aren’t being met.
Several of these problems may be combined, which is why a doctor needs to take a careful and complete medical, social and dietary history, examine the baby thoroughly, look at the mother's breasts, watch a feed and arrange lab tests.
Dr. Penny Stanway
June 2009
You can buy Dr. Stanway's book 'Breast is Best' here | 
Breastfeeding Twins
