The 28-year-old British actress flaunted her baby bump at the 2017 Oscars after party in a fitted black gown, reported Us weekly.
However, Egerton and Hartnett, 38, have yet to publicly announce their pregnancy.
The two even fueled engagement rumours when Egerton was spotted wearing a diamond ring on her ring finger.
The pair, who co-starred in 2013 film "The Lovers", have been together since 2012 and are already parents to a little girl, whom they welcomed in November 2015.