Test weighing before and after feeds

What is test weighing?

In the 1950s, during the era of the ‘scientification of motherhood’, it was common for babies in developed countries to be weighed before and after a feed, known as test weighing. This practice shouldn't be routinely practiced with term infants because of its lack of precision as a measure of either breast milk production or milk transfer.

• The density of human milk is approximately 1.03 g/mL. This means that one milliliter (mL) of human milk weighs slightly more than one gram (g), specifically about 1.03 grams. Because the density of human milk (1.03 g/mL) is very close to that of water (1.00 g/mL), the difference between 1 gram and 1 mL is minor (roughly 3%). This is why for most practical purposes, 1g is treated as 1 mL.

• In test weighing, parents don’t need to undress the baby, but just weigh baby before and after the feed.

Test weighing has been referred to at times as "objective measure of milk transfer" – but this is a false statement. No single indicator of milk transfer is objective. Assessment concerning milk transfer requires context.

Why is test weighing unhelpful?

As health professionals, we can significantly escalate our own and the parents’ anxiety about a baby’s weight by conducting pre- and post-feeds test weighing.

• Single feed test weighs are very unhelpful for breastfeeding, since any one feed is not indicative of average milk transfers overall, given how variable the amount of milk transferred in each breastfeed is, in successfully and exclusively breastfeeding pairs.

• Test weighing a feed and then measuring the amount of milk (‘residual pumping’) that can be pumped after the baby has finished breastfeeding is also remarkably unhelpful, and should never be used. Even women with very generous supplies may not be able to pump much milk.

When I have serious infant weight gain concerns which we need to turn around quickly if we are to hold off from formula use, then I will ask the parents to come back for a weigh within two or three days, or sometimes even the following day if that’s called for.

I don’t recommend parents use baby-scales to track their baby’s weight at home, but that they allow a provider to do this instead. We need to bear in mind that baby-scales can be calibrated differently, and that it is normal for a baby to gain very little one week, and then quite a lot the next (providing we are not dealing with a premmie or a baby in a vulnerable situation, when we need to track weight gains very closely). It is the trajectory over time, on the WHO percentile lines, that matters. If we weigh our babies too much, whether as health professionals or parents, we risk unnecessary worry and unnecessary formula supplementation.

What is 24-hour test weighing?

Twenty-four hour test weighing is used in research as an indicator of overall breastmilk transfer (taking into account other milk administered other than from the breast). But this can nevertheless be disruptive for the baby before and after feeds, especially the baby is already dialled up.

Even in this research setting, milk production with 24 hour test weighing may be underestimated by an average of 10% (range 3-55%) due to infant insensible water loss. Given that the upper end of the range of underestimation is 55%, this variability is significant.

Some clinicians send parents home with scales, requesting a 24-hour test weigh. I worry that this can be very disruptive for families, and yet doesn't yield any more information than careful history-taking and clinical assessment. Even though the baby is weighed before and after the feed fully clothed, the use of 24-hour test weighing may reinforce unhelpful infantcare behaviours because test weighing in the 24 hour context

• Rouses infants after a breastfeed, interrupting easy sleep

• Assumes that each infant breastfeed needs to be a 'meal'

• May elevate parental anxiety over how much milk is actually transferred each feed

• Potentially sets up a miserable day for that mother, who needs to stay at home for the test weigh, which may dial the baby up due to decreased sensory motor stimulation.