What is the ruling for a widow who is pregnant? Does her waiting period end immediately after she gives birth even if she delivers her child soon after her husband's death? On the other hand, is she to observe the same number of months as in the 'idda of a [non-pregnant] woman whose husband has died?

The 'idda (waiting period after divorce or the death of a husband) of a pregnant woman ends immediately after she gives birth whether she is widowed or divorced, provided the fetus is fully or partially formed. But if it is unformed, her 'idda does not end with delivery since the child is only considered a mudgha (lump of flesh) or 'alaqa (clot of congealed blood). God Almighty says, “The waiting period for those who are pregnant will be until they deliver their burden”
(Quran 65:4). The general implication of this verse denotes both divorced and widowed women.

Imam Ali and ibn 'Abbas (may God be pleased with them both) maintained that a woman is to calculate the period of her 'idda based on that which will occur last (the 'idda or giving birth). This opinion reconciles between the two Quranic verses,
“The waiting period for those who are pregnant will be until they deliver their burden”
(Quran 65:4) and “If any of you die and leave widows, the widows should wait for four months and ten days before remarrying” (Quran 2:234).

The majority of scholars maintain a different opinion, substantiating it with the same evidence used by ibn Mas'ud in that the former verse was revealed after the latter.

The Ruling

Based on the above, the 'idda of a pregnant widow terminates with childbirth.

God Almighty knows best.