One of the arguments proposed by advocates of permitting intentional abortion is that even if a fetus or embryo has a "right to life" (by virtue of being a separate human being), that right does not include a right to insist on another person's assistance. For example, if you need blood transfusions to live, that does not mean you have a right to demand blood from another person, nor to demand continued blood transfusions from someone who has begun to volunteer.
One possible response to this argument is to say that humans do have a duty to preserve the lives of others, and if your neighbor needs a blood transfusion to live, you do have a moral duty to provide that transfusion. The Westminster Standards, and Jesus' example of the Good Samaritan, suggest that such a duty exists - not as a "right to life" but as a duty to preserve life. That response really should suffice.
Suppose that we are wrong on this counter argument and that there is no general moral duty to inconvenience oneself to preserve one's neighbor's life. Still, there are clearly cases everyone accepts in which a person has a moral duty to support the life of someone else. We may be able to convince our friendly opponents of this with several examples:
1. The case of the car accident victim. Suppose you crash into another person and they are dying unless you act to save their life. In that case, I think most people would agree that you have some duty to try to save their life, even if it is inconvenient for you. This is somewhat analogous to the embryo or fetus, because the person is in the womb of his mother due to something his mother did. Therefore, she has a duty to save his life, even if it is inconvenient for her.
2. The case of paternal child support. Suppose you father a child out of wedlock. Most people seem to agree that the father has some duty to (at least) financially support the child, even if the child's life itself does not absolutely require such support. The justification seems to be either that the father acted by begetting the child and/or that the father has paternal duties toward the child. Much more so, a mother likewise acted, has maternal duties, and should minimally be required to save the child's life for a few months.
3. The case of a young infant. Suppose the fetus is born and consequently becomes designated an "infant." If a mother were simply to refuse to nurse (or otherwise feed) the child, we would view this as neglect and as murder if the child died from it. The same would be true if a single father refused to feed the infant. The justification here is pretty clearly parental duty.
Thus, in short, a general answer to this argument is that (a) we do have a general duty to preserve life and (b) that general duty is heightened in the case of parents with respect to their offspring. The only thing that remains to be seen is whether such a duty applies to offspring who have not yet offsprung.
But surely the duty is one that is based on the helplessness (or negative maturity) of the child, not on the self-sufficiency or maturity of the child. This can be seen from the fact that failure to feed a 30 year old son is not neglect, unless that son has a serious disability such that he cannot feed himself. An embryo or fetus is much more helpless and immature than an infant. Thus, the parental duty of support should be much greater for a fetus or embryo than for an infant.