The 41-year-old actress announced she was 15 weeks pregnant and is due to give birth in September 2017.
“I found out I was expecting in the second week of January and it has been very hard keeping it a secret until now.”
Titmuss took to Instagram to share the news with her followers, writing on 3 April: “Overwhelmed with joy to share the very special news with you... that I’m PREGNANT!”
She used the hashtags #miracle, #blessed, #privilege and #conception.
Titmuss moved to live in Hollywood, US, three years ago where she met her partner Welkom.
The couple got engaged in March 2016 and are set to wed in May 2017.