Why would you think that any particularly intrusive forced medical practive is ethical? Is such a policy really all that much more acceptable than mandating abortions?
- Paying taxes
- Child schooling
- Vaccination
- Health care
A population growth policy that targets birth is not so entirely different from those, as fundamentally different as it may feel, depending on your background. Giving people the option of either birth control or an abortion if they get pregnant would be one way to handle that. Have a fundamental disagreement with abortion? Use birth control. Have a problem with birth control too? Take it up with the state, just like in those items above.
I'm not advocating for this, I'm just pointing out it's not fundamentally different than other things we already do.
One possible argument might be that the social cost of that unborn child is so high that is it is morally reprehensible to have conceived it in the first place, and that such an action is a horrible crime.
Plenty of places suspend human right to imprison or even execute people who have committed crimes. It could be seen that such an action isn't even isn't even punitive, just corrective. Some might feel that the government simply removes the child before it is born, and then is lenient because the mother is not imprisoned for her indiscretion against society.
Other possible arguments could stem from the long term environmental harm that the child will have on the environment. If someone places higher value on nature than on human life this argument naturally follows.