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Speculation indicates the detonation of the atom bomb in the 70s is the reason that the women in the present die during pregnancy.

That makes absolutely no sense since Arron lived and Ji Yeun was conceived on the island after it healed Jin's sterility. There are many properties of the Island which remain unexplained but this may be related to whatever reasoning led to a condition in the treaty limiting the total population of the Island.

It's also worth noting that Juliet never did solve that, indicating this is just gonna be one of the Island's mysteries we don't have any reason to see the explanation for.

Surely this indicates that the Island was not real, and the reason that baby's cant be born is because everyone is dead on the island?

I agree with the unexaplained properties part, but: 1) Aaron was not conceived on the island, and by the time Claire came to the island she was past her second tri-mester, which is when women on the island died from pregnancy. 2) Sun had left the island before her second tri-mester. 3) What treaty? 4) There was a population limit on the island?

Additionally, Jacob and his brother were born on the island as well.

The Island was real and it got it explained in "The New Man In Charge". It had to do with the time trave/time manipulation experiments the Dharma Initiative were conducting in the 70's at the Orchid. They were using polar bears. There were strict instructions not to use pregnant females for the time manipulation experiments because of potential disasterous effects. Someone messed up and the effect was gestation for certain mammals on the island was disrupted by the properties of the island's electro magnitism afterwards.