The longest pregnancy in humans on record is 375 days (12.5 months).
The normal human pregnancy lasts around 280 days, 10 lunar (4 week months) months or slightly more than nine months. Traditionally a term pregnancy for a singleton is 38 to 42 weeks.
The longest duration known for a woman to be pregnant is 375 days, and it happened in 1945 when Beulah Hunter gave birth in Los Angeles to a healthy boy. She would have gone longer but doctors induced labor at this point. She had experienced a single pregnancy, as opposed to multiple embryos, such as with twins.