Alexandre and Ronaldo were born in Tufts Medical Center over the February/March period, when 35-year-old mother Lindalva went into labour just 24 weeks into her pregnancy.
At 745 grams, Alexandre is a third of the weight of his brother Ronaldo.
Lindalva says Alexandre's size - fitting into her husband's palm - did cause her to worry about his chances of surviving.
"They brought him to me to give him a kiss," she told ABC America. "He was crying, but he was very tiny and I was very scared. I said to my husband, 'He's too tiny'."
Doctors said the babies had about a 50 per cent chance of surviving.
"When you have a situation where one baby is born and labour stops, you are given this window of opportunity," Dr Steven Ralston told the Boston Globe.
"But it's a rare window. Most times, labour does not stop."
Data on delayed twin deliveries in the US is minimal. Only 14 cases have been recorded over 12 years, out of 96,922 deliveries, according to the journal of Obstetrics & Gynecology and two large medical centres in Virginia and Tennessee.
Both babies are still neonatal intensive care, but Lindalva is hoping to being them home later this month.
"They're my miracle babies," Lindalva told ABC America. "Ronaldo because he stayed inside, and Alexandre because he's a survivor."
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