The infant should be consistently gaining weight and growing. In newborns, a loss of more than 5 to 7 percent of birth weight warrants investigation. The use of IV fluids in labour tends to artificially increase the birth weight of the baby, and subsequently inflate the babys weight loss. Newborns should regain their birth weight by two weeks of age, and gain at least 150 g per week.If an infant is not showing these signs of growing well, other possible causes include:

Poor milk transfer due to tongue-tie, or cleft palate. Medical conditions in the infant, such as malabsorption of nutrients. Definition
A mother is considered to have low milk supply if she produces less breast milk than her infant requires. The term is used only after a mothers milk "comes in", which usually occurs around 30–40 hours after delivery of a full-term infant. Low milk supply is distinct from the scenario in which the mothers milk comes in later than normal but is thereafter produced in sufficient quantities; this is known as delayed onset of lactation (delayed lactogenesis II). Prevention
The first week after birth is a critical window for establishing milk supply. The Ten Steps of the Baby Friendly Hospital Initiative describe some maternity hospital practices that promote the development of a good milk supply, such as rooming in (allowing mothers and infants to remain together 24 hours a day) and having trained staff available to help mothers with breastfeeding. Supplemental formula or donor milk should be given only when medically necessary.
One aspect of supply regulation that has been identified is that breast milk contains a peptide called feedback inhibitor of lactation (FIL). When milk is present in the breast, FIL inhibits the secretion of milk. After a mothers milk comes in, a reduction in supply is inevitable unless milk is removed regularly and thoroughly from the breasts.Surgery or injury to the breast can decrease supply by disrupting milk ducts that carry milk from the alveoli, where milk is produced, to the nipple. Retained placenta results in high levels of progesterone, a hormone that inhibits lactation. Diagnosis
A womans belief that her milk supply is insufficient is one of the most common reasons for discontinuing breastfeeding. In many of those cases, the womans milk supply is actually normal. After a few weeks or months of breastfeeding, changes that are commonly mistaken for signs of low milk supply include breasts feeling softer (this is normal after 1–3 months), more frequent demands by the infant to feed, feeds becoming shorter over time, baby colic, the perception that the baby is more satisfied after being fed infant formula, and a slowdown in growth after three months.To evaluate whether milk supply is actually insufficient, qualitative parameters should be used such as the following:

By 3–5 days, of age, the infant should be stooling 3–4 times per day and urinating 3—5 times per day. By 5–7 days of age, there should be 3—6 stools per day and 4—6 urines. The infant should be alert, have good muscle tone, and show no signs of dehydration.