You're assuming an unwanted, unborn baby has value. Not only does it not have value, but it's actually a burden to society. Utilitarianism would consider the damage that unwanted, unborn baby will inflict on society and decide to abort it every single time. Social welfare, orphanage, and especially criminal costs are incredibly likely and incredibly high. The chance that the baby will amount to anything worthwhile enough to offset those costs is incredibly low for an unwanted baby and thus not worth the risk to society. Here in the US we have seen the criminal costs of outlawing abortion with the high crime rates of the 70's, 80's, and early 90's finally coming down in the last two decades due to legal abortion. Other countries like Romania know this equally well. That doesn't even begin to take into account the rest of the social costs of forcing unwanted babies to be born.
First, your statement that abortions have decreased crime in the US is based on one of the most flawed and controversial studies in the scientific community [1]. Correlation does not imply causation.
Second, Utilitarianism roughly translates to "the greatest good for the greatest number." If a pregnant, utilitarian woman was granted with the power of foresight, she would abort her child only if he/her was to provide a negative net utility to society. If the child was to provide a net benefit to humankind, she would not abort.
Third, your assumption that unborn (and born) children are a burden to society is correct, but that initial investment is small with regards to the average net "benefit" a grown human creates. It must also be noted that the vast majority of humans benefit humankind through their work (although some have greater impact than others).
No person can see the future, however, so most utilitarians would never abort their children, as the probability their offspring benefit humanity as a whole is greater than the chance that they destroy value.
Frankly, I doubt any woman considers the ethical implications of abortion when they undergo one, and are primarily concerned with family, relationship, and personal problems. If one thinks beyond personal convenience, and look at the bigger picture, abortions are morally unjust from almost every popular ethical system.
Think hard about the choice your parents made by not aborting you. Do you think they made the right decision? Whether you're old, young, rich, poor, hopeful or hopeless, I imagine you'll answer yes.
