The sweet snap shows Julia's husband gently placing his hand on her stomach, wedding ring proudly in place, while Julia cradles her baby bump from below, her wedding and engagement ring just visible in the picture.
Reports say that the couple chose to wed in secret, in an intimate ceremony attended by just two close friends as witnesses. Julia captioned the happy picture on Instagram: 'Who doesn't love a shotgun wedding?'
The 36-year-old Hollywood star announced her engagement to Preston in January, sharing a sweet snap of the couple's interlinked hands with the caption: 'Best Christmas Ever!'
Julia later revealed that her partner Preston has chosen Christmas Eve to propose, while the couple were staying on the beautiful Isla Grande in Colombia.
'All around, it was a really wonderful trip. We stayed on an island that was really beautiful and there was a lot of great snorkelling. The city was really nice, too, but because I live in the city the natural scenery was really great,' Julia told People magazine. 'That's where we got engaged.'
Following the news of their engagement at the beginning of this year, Julia confirmed in June that she was expecting her first child with Preston in a low-key announcement via her agent.
It is thought that Julia is due in the next few weeks, and the mum-to-be revealed her excitement to Stylist recently.
'I know it's something that women have been doing for ages. but for me it's the first time and I'm so happy and so excited,' she said.
At the time of her pregnancy announcement Julia said: 'We're really slow on the wedding planning', so it would have come as a surprise to many of her fans when Julia revealed that she and Preston had tied the knot at the weekend.
Sharing the news in August that she was 'nearly eight months pregnant', fans will be waiting to hear the news of the baby's birth in the next few weeks.
Amongst secret planning for their intimate wedding ceremony, Julia also said that she and Preston had settled on a name for their new arrival - but wouldn't admit what the name was at this stage.
'We have a name that's been passed around and it's stuck, and now we're used to using it. So yeah, we have a name and both sets of grandparents aren't thrilled about the name - but I think they'll come around.'