Same dose, opposite answers on cramps in pregnancy
At 300mg a day, one trial had 86% of pregnant women halve their cramps against 60.5% on placebo; another at the same dose found no gap at all. Eight trials could not be pooled.
What the study found
Two trials at the same magnesium dose reached opposite answers
One trial gave pregnant women 300mg of magnesium a day, and 86% halved their cramp frequency against 60.5% on placebo. Another, at the same dose, saw both groups improve by about a third and no gap between them. The Cochrane review of eight trials and 576 women could not pool them.
The caveat · Two different magnesium compounds
The two trials used different magnesium compounds, so the split may be about which form sat in the capsule rather than about magnesium. Both groups improving by a third in the null trial points at expectation and the natural course of cramps doing much of the work. In the oldest positive trial, blood magnesium never actually rose, which makes it hard to credit magnesium with the improvement. And the trials counted cramps so differently that no single number can be drawn across them.
What we don't know
The two trials used different magnesium compounds, so the split may be about which form sat in the capsule rather than about magnesium. Both groups improving by a third in the null trial points at expectation and the natural course of cramps doing much of the work. In the oldest positive trial, blood magnesium never actually rose, which makes it hard to credit magnesium with the improvement. And the trials counted cramps so differently that no single number can be drawn across them.