Allen Leech knows the sex of his unborn baby.

The 'Downton Abbey' star is set to welcome his first child into the world with his wife Jessica Blair Herman later this year and, although they've found out whether they're having a little boy or girl, he wants to keep the gender a secret until the bundle is born.

When asked whether they know what they're having, Allen told 'Entertainment Tonight': ''We do. But we're not telling ... I want four [kids]. And Jess' response to that probably can't be broadcast on national television. So she's like, 'Yeah, good luck. See you there.'...Fingers crossed everything goes brilliantly with this and we'll see then.''

The 38-year-old actor can't believe how well his wife has taken pregnancy in her ''stride'' and is amazed with how the human body adapts to grow a mini human.

He explained: ''It is amazing and Jess is doing a phenomenal job.

''She's just taking in her stride. So it is incredible and you're sitting there going, 'Look at how your body is changing!' And then sometimes you're [like], 'Isn't it amazing?!' and then sometimes you go, 'What!' And I mean [that] in a good way.''

However, the hunk has realised he has to be more considerate about the way he speaks about her body now that she's got a blossoming baby bump.

He said with a laugh: ''Really listen to the question, and be careful how you answer. That would be my advice. Really listen.

''If she says, 'Do I look fat in this, that's not what she's actually asking.''

The couple tied the knot at a ranch in California's Santa Ynez Valley in January after three years together, but the ceremony didn't go according to plan as it ''peed rain'' and the hired band were unable to play.

But luckily their guests Darren Criss and Lea Michele stepped in instead.

Allen said previously: ''We had blankets and heaters around but it was so wet, Jess had to be carried down the aisle by the groomsmen!

''People were drinking hot toddies when Darren started playing 'All You Need Is Love' and everyone joined in.''

The couple met at a party in 2016 and the hunky actor instantly fell for Jessica when she fabricated a long history between them.

He recalled: ''There was this moment when we were chatting just the two of us, and this guy walked past, really hammered, and said, 'This - this is what I want. How long have you been married?'

''Without missing a beat, Jess replied, 'Six years', and started making up this story.

''I just looked at her, giving this guy marriage advice and thought, 'She's pretty cool.' ''