How small are newborn tummies really, and why do they need to suckle so much?
Newborns expect a lot of suckling at the breast during the first days (and nights) of life
From an evolutionary perspective, a newborn expects a great deal of suckling at her mother's breast in the first days of life.
It doesn't seem to be hunger that makes your newborn cry for the breast in the first two or three days of life - it's the need for sensory motor nourishment, the need to be up close to your body, drinking in the wonderful, wrap-around sensory motor experience of suckling at your breast. It is difficult to separate out a baby's need for physical contact and the sensory motor experience of suckling from the baby's need for milk. From an evolutionary perspective, these two elements can't be separated, as (in best possible circumstances) the sensory motor drive will also meet the infant's caloric needs.
Newborns don't need a lot of milk in the first days after birth
Your exclusively breastfed baby might take no more than one to five millilitres of breastmilk each time she suckles at the breast in the first day or two of her life. Despite the very small amounts of milk transferred early on, which is completely normal, your baby needs the opportunity to suckle at your breast very frequently from that very first day if you possibly can, to establish your best possible milk supply down the track.
The newborn stomach might be surprisingly small in size - but this is not relevant to the baby's caloric needs
Some health professionals illustrate that it's not the need for milk which primarily drives suckling at first, by offering comparative estimates of the size of a newborn's stomach. These estimates are inaccurate. You might have seen charts with these estimates, but they are not science-based, and might be misleading. For example, well known neonatologist Dr Nils Bergman calculated in a 2013 publication that a baby's tummy at birth is more like 20 mls in capacity.
A full-term baby has been swallowing 500-1000 ml of amniotic fluid every day. Newborn stomach size is highly variable from birth. The stomach is also designed to stretch. Stomach capacity does not determine a baby's needs for milk - the baby's metabolism does.
Tables which compare a baby's tummy size with items like a cherry on day 1, walnut on day 2, apricot day 7, and hen's egg day 28 aren't accurate, and don't consider the many factors which interact in a mother and baby's biological system to ensure baby's adequate caloric intake in the first days after birth. Imagined stomach volumes are not a helpful indicator.
Recommended resources
You can read a blog on newborn stomach size here.
Selected references
Bergman, N.J. (2013), Neonatal stomach volume and physiology suggest feeding at 1-h intervals. Acta Paediatr, 102: 773-777. https://doi.org/10.1111/apa.12291
