I am a foreign student and currently applying to several PhD programs in the US. My GPA in the 6th semester was a lot disappointing, and pulled down my overall GPA and ranking significantly. The reason is that I got pregnant, by accident. I want to explain this situation in my statement of purpose or address an email to the admission committee, so I want to know how Americans, especially professors, will normally react to this explanation? Will they regard it as understandable mistake or unforgivable sin?
I'm going to side-step this question by simply saying: in PhD admissions, grades are not nearly as important as one might think. Far more important are 1. your letters of recommendation and 2. tangible evidence that you are capable of being creative and productive in a research environment (e.g., class projects, publications if you have them, etc.). One semester of bad grades is not going to kill your application, especially if the rest of your grades are strong.
Some even broader advice is: draw attention to the good rather than the bad. Compare the two statements:
(A) "Despite my poor performance in area X, I believe that I am still a strong candidate for the program in (your field of study)."
(B) "My strong performance in area Y demonstrates that I am well prepared for a research career in (your field of study)."
Both statements communicate the same idea ("I am qualified"), but (B) draws attention to the good, whereas (A) draws attention to the bad. Don't draw attention to the bad. If people think something is bad, they'll think it's bad no matter what you say about it. Get them focused on the good stuff instead.
I fully agree with Dnuorg Spu's answer (emphasize strengths, not perceived weaknesses), but it could be worth a brief, vague mention of this issue somewhere in your application. For example, something along the lines of "You'll notice that my grades dipped briefly during my 6th semester before returning to normal. This was due to a personal issue that has been resolved." The idea is to reassure the admissions committee that it's not a sign of flakiness or ongoing difficulties. I'm not sure whether such a statement makes a real difference, but at least you would be stating on the record that your 6th semester was anomalous for a reason unlikely to be repeated. Hopefully your 7th semester grades will be available to the committee and will demonstrate that you are back to normal.
I would recommend against going into more detail than this, however. There's too much of a risk of distracting people, since the whole topic (young mothers raising children, adoption, abortion, etc.) is emotionally charged, especially in the U.S. You don't want this issue to be the most attention-getting or memorable part of your application. It would probably not hurt your chances, but what happened is none of the committee's business and the details are not relevant for whether you should be admitted, so it's safest to keep the discussion focused on academics.