Story highlights Gerardo Hernandez was the head of the spy ring known as the Wasp Network

Since 2001, he was being held in a U.S. prison; he was released in a prisoner swap

Havana has been abuzz since his wife in Cuba showed up pregnant during news coverage

Havana, Cuba (CNN) It might be the most bizarre of the closely guarded secrets from last week's historic agreement between the United States and Cuba: How did the leader of a Cuban spy ring serving life in a California federal prison manage to impregnate his wife 2,245 miles away in Havana?

As part of the most significant diplomatic breakthrough between the United States and Cuba in more than 50 years, a prisoner swap was made. To uphold its part of the bargain, the U.S. released three Cuban spies, including Gerardo Hernandez, the head of the spy ring known as the Wasp Network.

Hernandez had an ear-to-ear smile Wednesday as he arrived at Havana's Jose Marti International Airport. State TV showed Hernandez as he was greeted by President Raul Castro and then embraced and kissed his wife, Adriana Perez.

Cubans watching the nonstop coverage of the prisoner swap were shocked when the cameras zeroed in on Hernandez hugging Perez. She was obviously in the late stages of a pregnancy that had no easy explanation.

Not only had Hernandez been serving a double life sentence, but his wife also worked for Cuba's intelligence services and was banned by U.S. officials from visiting her husband in prison, according to the Cuban government.

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