In plain words
The milk-to-plasma ratio compares how much of a substance is in breast milk with how much is in the mother's blood. A ratio below one means less of the substance reaches the milk than stays in the blood. A ratio above one means more does. Caffeine is one example with a ratio below one. Clinicians use the ratio together with the baby's age, size, and feeding amounts to judge exposure. It is one factor among several. It does not settle whether something is safe on its own. For any medicine, supplement, or herb, ask your provider or a lactation resource for the details.
The deeper version
The milk-to-plasma ratio is the steady-state concentration of a drug or chemical in milk divided by its concentration in maternal plasma. Passive diffusion, ionization, protein binding, and lipid solubility determine the value. Low molecular weight, low protein binding, and non-ionized forms tend to give higher ratios. For caffeine, the ratio in lactation is about 0.6 to 0.7, so milk levels are somewhat lower than plasma levels (National Library of Medicine, ). Infant exposure also depends on the infant's age, clearance, and feeding volume. The controversy is that many ratios come from small studies and describe averages that may not apply to any one person.
Where it comes up
- Alcohol, caffeine, and breastfeedingFeeding Your Baby
- Coffee and caffeinePregnancy: Eating
- Medications and breastfeeding: how to checkFeeding Your Baby
- Herbs and teas postpartum: what's traditional, what's knownNourishment and Traditions
What mothers here say about milk-to-plasma ratio
No one has posted under this term yet.
Sources
- National Library of Medicine — LactMed: Drugs and Lactation Database, Caffeine entry
- Hale TW — Medications and Mothers' Milk (annual edition)