Shapur II of the Sassanid Empire was crowned in utero by having the crown placed on his mother's belly, he ended up ruling for 70 years.

This was the case with Alexander the Great in 323 BC, his wife Roxane was six to eight months pregnant with Alexander's child. After Alexander died his generals basically carved up the Empire into pieces, but Antipater was named regent of Macedonia. The newborn infant of Alexander was to become king when he came of age, but the kingship was shared with Alexander's half-brother Arrhidaeus for awhile. Antipater died, and in 316 BC Cassander took over power in Macedonia. Killing Alexander’s mother Olympia, and on the orders of Cassander, Claucias also killed Roxane and Alexander the Great’s son and heir.

I believe this happened in 1316 during the reign of Louis X of Capetian France ('the Hutin', son of Philip IV 'the fair'). His wife, Clemence of Hungary gave birth to Jean I (the Posthumous) after his death. Jean I himself only survived 5 days, and was promptly usurped by Philip V of France (Louis X's brother) This was done over Louis' daughter Joan. Joan's legitimacy was under question due to an earlier affair by Louis X's first wife Maurguriette of Burgandy (known as the Tower of Nesle affair). Philip used the precedent of Salic law to achieve this.

As a result of this, and due to Philip's own death in 1322 heir-less, it resulted in the coronation of Charles IV, youngest brother, who also died heir-less, and due to the marriage of Isabella (Philip IV's eldest daughter) to Edward II of England, precipitated the Hundred Year's War - as Isabella's son Edward III considered himself King of France (though eventually the English house of Plantagenet lost to the house of Valois - members of the dynasty formed from Phillip IV's younger brother, Charles de Valois)

Alfonso XII of Spain died in 1885 while his son was still in utero. The son was born in 1886 and crowned as Alfonso XIII.