Rachel Hunter is excited to become a grandmother. Photo / Paddy Foss

Rachel Hunter is going to become a grandmother for the first time.

The Kiwi model’s son Liam Stewart, 28, whom she shares with ex-husband Sir Rod Stewart, 78, has announced he is expecting his first child with his partner Nicole Ann.

The couple announced the news by posting a black and white Instagram video showing Liam holding their first child’s ultrasound image with the message: “Mom and Dad – Baby Boy joins us in May”.

The pair are also in the clip at a gender reveal party, bursting open a barrel of confetti in front of a Christmas tree.

Hunter, 53, wrote below the post, “Love you guys so excited the best Mum and Dad ever!!! I can’t wait! Finally I get to tell everyone. Didn’t have my glasses on and put up the wrong heart!!”

Stewart’s current wife Penny Lancaster, 53, also commented on the clip, ”Beautiful, I can’t help watching it over and over again.”

The Auckland-born model and actress married Stewart in 1990 and shares daughter Renee, 30, and Liam with him. The couple divorced in 2006.

Among the famous faces to also remark on the news was singer Natalie Imbruglia, 47, who said: “Omg!!! Congrats.”

Sir Rod has eight children with five women, including sons Alastair and Aidan with third wife Lancaster.

He also has Kimberly, 43, and Sean, 42, from his first marriage to actress Alana Stewart, 77, and his daughter Ruby, 35, is from a relationship with 63-year-old model Kelly Emberg.

Sir Rod’s first child was Sarah Streeter, 59, who was raised by her adoptive parents Gerald and Evelyn Thubron and he is grandad to Kim’s daughter Delilah, 11. The Maggie May singer last year mourned the death of his brothers Bob and Don, who died within months of each other.

- Additional reporting, NZ Herald