Allah said:

(...and it is not lawful for them to conceal what Allah has created in their wombs,) meaning, of pregnancy or menstruation periods. This is the Tafsir of Ibn `Abbas, Ibn `Umar, Mujahid, Ash-Sha`bi, Al-Hakam bin `Utaybah, Ar-Rabi` bin Anas, Ad-Dahhak and others.

Allah then said:

(...if they believe in Allah and the Last Day.)

This Ayah warns women against hiding the truth (if they were pregnant or on their menses), indicating that they are the authority in such matters as they alone know such facts about themselves. Since verifying such matters is difficult, Allah left this decision with them. Yet, women were warned not to hide the truth in case they wish to end the `Iddah sooner, or later, according to their desires. Women were thus commanded to say the truth (if they were pregnant or on their menses), no more and no less.