The obvious question is what is a pregnant woman who is in such distress that she needs to sit down immediately doing on a train or bus? Why is she there? If it is her choice, she deserves no consideration. If not, where is the husband? Yeah, I said husband, not baby-daddy. If he doesn't care, why should I? The innocent woman in dire circumstances beyond her control is a very rare event, so shaming and discouraging such behavior should be the norm. Hence, the answer is No, excepting a true and obvious medical emergency. Convince me I'm wrong.