Meryl Streep is set to become a grandmother for the first time

The couple announced they had gotten engaged during a holiday to Croatia in August, with Mehar adding a picture of the loved-up pair on Instagram, and simply captioning the post: "My fiancé."

Prior to meeting her other half, Mamie was married to the actor Benjamin Walker - whom she met on the set of 'Dangerous Liaisons' in 2008 - however they split in 2013, after just two years of marriage.

The 'Florence Foster Jenkins' actress - who also has Grace, 32, Louisa, 27, and 39-year-old son Henry with husband Don Gummer - has maintained a largely private life, and it's expected that once the new addition to the family arrives Mamie will follow suit.

Mamie can look forward to making her own memories with her little boy or girl as they grow.

In a rare interview with in 2016, the 'True Detective' star recalled how her mom - who will next be seen portraying a strict mother in the second season of 'Big Little Lies' - saved her from an embarrassing Halloween outfit mixup as a teenager.

The actress dressed up as Charlie Chaplin only to find the rest of the party guests not in costume.

She recalled: “Mum, without blinking, gave me her leather jacket, put it on over the costume and wiped my make-up off.

"It was that transition [from childhood to adulthood] that’s so difficult for kids, made within the space of 20 minutes. And she was there on both sides of it. She’s a great mother.”