I am somewhat skeptical of this hypothesis. The market may be less saturated, but AFAIK also more competitive. A cupcake aficionado may read ten blogs on the topic, and Google can send lots of super-niche long-tail traffic if you rack up enough posts — but it's unlikely that many people will buy more than one or two fertility calculators in their lifetime, and they'll most likely just buy the first one they see that looks OK.