All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
What the Quran states that the period of pregnancy is six month is correct and you should not say that it is wrong, because the pregnancy has its maximum period and its minimum period. The maximum period of pregnancy is nine months as it is well-known, and its minimum period is six months. There are some women who give birth when she is only six or seven or eight months pregnant, and the born child lives. Allaah, The Most Exalted and The Most High, clarified to us the minimum period of pregnancy because there are some religious rulings that apply to it, like establishing the lineage of a child or denying it, inheritance, divorce, the waiting period and the like. Among these issues, is that if a woman gives birth to a child five months after marrying her, then the new-born is not traced back to the husband.
Therefore, it is not permissible to describe what is mentioned in the Quran as being wrong. Rather, what the Quran states that the minimum period of pregnancy is six months is confirmed by modern science, that the fetus may be born at the age of six months and it can live, and this is what is called a premature baby. Moreover, medical evidence proves that the fetus that is born before a full six months period does not live.
Finally, we advise you to avoid debating with the non-Muslims if you do not have enough religious knowledge in to refute their misconceptions.
Allaah Knows best.