The original Japanese version of the series ran for 52 episodes while the English version ran for a reduced 51 episodes, as the first two episodes were combined into one, resulting in the omission of the entire backstory of the main antagonist. This creates issues when the antagonist appears later in the series, and the anime refers back to the events which were cut. (Source: Astro Boy Wiki and Wi

The original Japanese version of the series ran for 52 episodes while the English version ran for a reduced 51 episodes, as the first two episodes were combined into one, resulting in the omission of the entire backstory of the main antagonist. This creates issues when the antagonist appears later in the series, and the anime refers back to the events which were cut. (Source: Astro Boy Wiki and Wikipedia)