The waiting period of the woman whose husband died

And those who are taken in death among you and leave wives behind - they, [the wives, shall] wait four months and ten [days]. And when they have fulfilled their term, then there is no blame upon you for what they do with themselves in an acceptable manner. And Allah is [fully] Acquainted with what you do.

[Qur'aan 2:234]

When the husband of a woman dies, she is to complete a waiting period of four months and ten days. The rule is applicable to woman regardless of if the marriage had been consumated prior to the death of the husband or not. The Verse does not apply to a woman who is pregnant from her husband and then he dies. Her waiting period is mentioned in the following Verse:

"...And for those who are pregnant, their term is until they give birth..."

[Qur'aan 65:4]

Once the pregnant woman gives birth her waiting period is over. The following Hadiith also shows this

Narrated Abu Salama:

A man came to Ibn 'Abbas while Abu Huraira was sitting with him and said, "Give me your verdict regarding a lady who delivered a baby forty days after the death of her husband." Ibn 'Abbas said, "This indicates the end of one of the two prescribed periods." I said "For those who are pregnant, their prescribed period is until they deliver their burdens." Abu Huraira said, I agree with my cousin (Abu Salama)." Then Ibn 'Abbas sent his slave, Kuraib to Um Salama to ask her (regarding this matter). She replied. "The husband of Subai'a al Aslamiya was killed while she was pregnant, and she delivered a baby forty days after his death. Then her hand was asked in marriage and Allah's Messenger (sallallaahu alayhi wa sallam) married her (to somebody). Abu As-Sanabil was one of those who asked for her hand in marriage".

[Sahih al-Bukhaari Hadith No. 4909]