Robitussin is heralded by many as a miracle fertility aid. A quick Google search and you will find plenty of women claiming that Robitussin is what helped them get pregnant. A Globe and Mail article billed it as "a $5 bottle of hope."
How can Robitussin possibly help me get pregnant?
Robitussin's main ingredient, Guaifenesin, is used to thin mucus. It's intended use is to clear chest congestion (phlegm). Guaifenesin has the same effect on all types of bodily mucus, including cervical mucus.
In some cases, a woman might produce too much mucus or have too thick of mucus. In these cases it is believed that Robitussin may help thin out that mucus, allowing sperm to travel through the mucus and *hopefully* fertilize an egg.