A woman becomes a second-class citizen as soon as she becomes pregnant because her human and civil rights become less important than the rights of the fetus. Here's how it works.
Pregnancy is a life-threatening condition. It is more risky to be pregnant than to not be pregnant. At any stage of a woman's pregnancy, it is less risky for her to terminate the pregnancy than to undergo childbirth.
In this country, even if you voluntarily assume the life-threatening risk of a condition (buying a ticket for a plane that might crash, ordering your eggs or steak undercooked, strapping yourself into bungee cords for a jump) you can always change your mind and back out of your decision (not getting on the plane, not eating the eggs or steak, not bungee jumping off the bridge). That is to say, in this country, we don't force people to go through with risky things, no matter how enthusiastic we were for the experience when we signed up for it (order you onto the plane at gunpoint, force-feed you the eggs or steak, push you off the bridge).
But we do, to one degree or another, if the life-threatening risk taken on is pregnancy. This is the only such situation in American law. And the given reason that we do so is because of the state's interest in the rights of the fetus. In other words, we force women to continue a particular life-threatening condition to its conclusion solely because of the rights of another being. And this being isn't even another citizen; it can't own or transfer property, it can't vote, it can't serve in the military. But as soon as a woman is pregnant, it causes her to lose the fundamental human right to direct her own healthcare and her own future. The option of adoption aside -- because the reality is that few women who bring unintended pregnancies to term go through with an adoption plan -- women are then routinely forced to undergo 18+ years of the legal, emotional, financial, and physical consequences of motherhood. Never mind the lack of state-funded daycare or the fair distribution of free public education for these children whose rights the state supposedly had such an interest in before they were born.
The only class of people in this country who are ever forced to go through the riskier of two options in any situation, because of the supposed rights of another party or class of beings, are women who are pregnant. If that isn't a solid definition of slavery, I don't know what is.