The actress, who usually has curly hair, shared a picture alongside her husband Jeremy Brier to her Instagram.
She captioned the photo "Thank you to everyone for the most fabulous messages and gifts...... 40's not so bad when I have this by my side...."
Fans rushed to comment on the picture, commenting how nice Louisa's hair looked straight.
One joked: "Does your hair magically turn straight when you hit 40?" with a winking face.
One commented: "Happy Birthday! You look so different with your hair straight! Beautiful straight or curly x"
Happy Birthday! You look so different with your hair straight! Beautiful straight or curly.
Another commented: "Love the straight hair, you look years younger with it straight. Birthday wishes."
A third put: "Wow you're hair is amazing."
A fourth added: "Happy birthday @louisaclein hope you've had a really lovely day. You look lovely with your straight hair."
Louisa began playing school teacher Maya Stepney in Emmerdale last year.
Days after he turned 16, Maya began sleeping with the schoolboy.
Eventually Maya was caught and David called the police on his ex-girlfriend.
Maya was recently sentenced to 12 months in prison.Louisa, who has three children with her husband, is also close friends with Judge Robert Rinder.
She also revealed that Robert introduced her to Jeremy, who she recently celebrated eight years of marriage with.
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