The placenta uses active transporters to move iron, folate, and calcium against concentration gradients toward the fetus, so the fetus is served first.The placenta is not a passive sieve. For iron, folate, , and calcium, it uses active transporters to move nutrients against concentration gradients toward the fetus. In normal-to-moderate deficiency, the fetus is prioritized — fetal blood calcium even exceeds maternal, held by calcium pumps.
2 · The cost: mother is drawn down
The placenta does not stop pumping when the mother is short, so her iron, bone calcium and can be drawn down.Fetal priority means the mother can be drawn down: drops, bone calcium mobilized, transferred from maternal brain and membranes. This is why iron and calcium stores should be topped up before pregnancy, not supplemented after deficiency appears — the placenta does not stop pumping because the mother is short.
3 · Capped nutrients: mother short, fetus short
The placenta cannot conjure nutrients such as vitamin D and iodine, so when the mother is short, the fetus is short too.Some nutrients the placenta cannot conjure. Vitamin D: fetal tracks maternal almost linearly (r ~ 0.9); the fetus makes none. Iodine: before ~20 weeks the fetal thyroid depends entirely on the mother. If the mother is short, the fetus is short — an active pump cannot pump what the mother does not have.
4 · What the placenta cannot block
Alcohol crosses the placenta freely and methylmercury is actively concentrated by it, so the placenta blocks neither.Alcohol crosses freely: 1-2 hours after drinking, fetal BAC roughly equals maternal, and the fetus cannot metabolize it, so it soaks longer. Methylmercury is worse — cord blood exceeds maternal (ratio ~1.7); the placenta actively concentrates it. So avoiding alcohol and high-mercury fish is more concrete than any supplement.