The couple, who got engaged earlier this year, are expecting their first child together after two years of dating, but the baby news didn't come as a complete shock to the pair as they had already discussed starting a family together.
A source told People: "They are both beyond ready to be parents. They've been talking about having children for nearly a year. They are so excited for that next step."
Mila, 30, and Ashton, 36, are said to be keen to tie the knot next month before their new bundle of joy arrives later this year.
The source explained: "I think they'll be traditional and get married before the baby is born. Ashton wants this marriage and relationship to be perfect."
However, the duo aren't planning on having an over-the-top ceremony and only want their immediate friends and family present when they exchange vows.
A source said previously: "They are planning to get married in LA in April. They just want it to be made legal so are happy with a registry office.
"Mila, especially, wants it to be very low-key, with just their family there and a BBQ in their garden afterwards. She doesn't want to do anything showy or over the top."
Ashton began dating the brunette beauty in 2012, following his split from his first wife Demi Moore, but they have known each other since Mila was 14.