Before an in vitro fertilization (IVF) procedure, all embryos sit for a few days in a dish of chemicals that help them grow. It turns out that differences in the type of medium used might affect the success of the procedure and the characteristics of the resulting baby, STAT reports. IVF experts have long suspected that differences in the roughly 20 commercial media are important, but this is the first study to randomly assign embryos to one of two different types of medium, known as HTF and G5. Differences were relatively small, but significant: Couples who got G5 had a higher pregnancy rate and had slightly heavier babies. The finding, published yesterday in Human Reproduction, may lead fertility experts to demand that media manufacturers disclose exactly what’s in their products.