Three things change the moment caffeine enters a pregnant body. Your liver slows down: the enzyme that clears caffeine loses two-thirds of its speed by the third trimester, so the half-life stretches from about 3 hours to 10 ¹. It crosses to the baby within minutes, and the fetal liver can't metabolise it, so your blood level is the baby's. And it narrows the placental vessels that carry oxygen and food, so slightly less gets through.

The effect that holds up everywhere is birth weight. Every 100 mg a day tracks to roughly 20–30 grams lighter ², and 200–300 mg raised growth-restriction risk by about half ³. The 2022 study using genetics to sidestep confounding found no causal link to miscarriage but confirmed the birth-weight one ⁴, so it's real rather than an artefact. Measured in blood, babies ran smaller even around 36 mg a day ⁵. There's no clean dose where the signal disappears.

What you're betting is small but stacked on someone who can't consent. A hundred grams of birth weight isn't visible in any one baby; it's visible in the tail, the babies who'd have been borderline. The upside of the coffee is real but small; the downside is small but real, and lands on the baby. That asymmetry is the case.

Count milligrams across the whole day, from every source. Not cups of coffee.

The cleaner rule: one cup, before noon, most days; herbal or decaf the rest. Well under the ceiling, and simple enough to keep when you're exhausted.

The biggest wins are silent: the placental vessels staying open, the "small for gestational age" note never written. That's relief rather than a transformation. What you'll actually feel is sleep, within a week. Those 1am wake-ups your app calls normal are partly the afternoon latte still active 11 hours later. Less reflux, fewer palpitations, a calmer baseline follow within days. After delivery your clearance returns to normal in a couple of weeks.