I think the actual duty begins as soon as she has the potential of being pregnant. Take maternity vitamins ahead of time, stop drinking immediately if the condom breaks, etc. Many women think they will abort, but then find that they won't when the time comes. It would be awful to poison a fetus if the decision to abort was aborted.
This is surely too stringent. If this is the case, anyone (if female) who has had sex in the past few months, regardless of the amount of contraception they have been using and the fact they will abort a foetus, has a significant 'duty of care'. They are, you say, morally
obligated to be teetotal, physically fit and full of specific natal supplements because of their duty to a possible foetus.
On what grounds do you possibly support such an obligation? It is not anything to do with making people's
lives as good as possible, that should be clear. Although it is the case that some people do not abort although they thought they would, in some case we have people whom are absolutely certain that they would abort if they became pregnant. Unless you are committed to the belief that a foetus has a (valuable*) life from the moment of conception, no one's
life is at stake here. If one aborts a foetus before it can be characterized as alive, and one is sure that that is the course of action you will take, that you take 'sensible natal precautions' has no effect on anyone's life. The foetus will never be a child whom would suffer from your actions. There is simply no life to be affected.
It seems you might be making a more epistemic claim; nobody is
certain enough that they will have an abortion to warrant the
risk. The risk here is the damage drinking and so on might do to a child. But this claim is false. Some people are very certain indeed they will have an abortion if they became pregnant. They may also, due to their use of contraception, be very certain they are not pregnant. The certainty they hold here is no less sturdy than the certainty we require to warrant other risks. In many cases, it may be more so. When I drive a friend (or a child!) to an appointment I am certain enough that I will not crash to warrant the risk. However, my certainty here is not absolute; it is definitely the case I could crash and greatly hurt both myself and my friend (or child). It is surely plausible that one could be as certain one would have an abortion/was not pregnant as one is that one will not crash one's car in a given journey. If the latter warrants taking a risk (and a greater one than the former; possibly killing someone) however could the former not?
I do not see any other plausibly justification one could use to support the obligation you propose. Certainly, we cannot claim that sex is an inherently reckless act and thus engaging in it bestows on one a special obligation. If proper precautions are used, and definitely if one has is committed to abortion in the instance of pregnancy, I fail to see how sex would be anymore reckless than driving my friend to his appointment.
A more plausible obligation needs to be much less stringent. It surely needs to be probabilistic; one has a duty of care to a foetus once there is a certain probability that that foetus will become a person. Your condition could be re-constructed as describing that 'certain probability' as anything above zero. As we have seen, this is unsupportable. The 'certain probability' may be fairly low, but should be in line with our other assessments regards risk, to ourselves and other.
*I do not think it necessary to assume all life is valuable. The life of a tubercle bacillus (tuberculosis bacteria) does not seem at all valuable. The life of some humans also does not seem value; I fail to see how the life of a human in an irreversibly vegetative state is valuable (it is certainly not valuable to them). A valuable life is surely one where the holder, at the very least, has some preferences. This is not the case with a foetus.