Maybe they're just large women? Alternatively you could be misinterpreting it, because it's statistically very unlikely that pregnant women could "have a habit" of hitting on you, so you're probably just taking what they say entirely wrong, or you've had one pregnant woman hit on you and decided they must all do it, although I personally think the former is more likely (Gods this sentence is long)
Frankly, it's neither stupid, nor dangerous, nor pointless, so count me out.