I must concur with the others...But I must wonder, because I have read numerous stories of women who had no idea they were pregnant because they still experienced their monthly cycle - heck I read of a lady who didn't know she was pregnant until she went into labor! Of course the child was born 4 months premature, but it's the principle of the matter.
If a woman is pregnant but still bleeds as normal every month, that would mean that the lining of her uterus is being shed. If that was the case, anything implanted would go with the blood, wouldn't it? And bleeding outside of spotting during pregnancy is usually an indication of miscarriage. And also if the woman was having regular periods during pregnancy, that would mean she was ovulating and that conception could occur again with unprotected sex...But that obviously is not possible.
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So to answer your question,
no it is not possible to conceive a second child while already pregnant, unless your body is really really screwed up.
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Scaredbaka