Heather du Plessis-Allan: This is our royal baby

Jacinda Ardern is now, quite possibly, virtually untouchable.

Given the Prime Minister's warm disposition and resulting popularity, it was already tough for her political opponents to land blows on her. Now that she's having a baby, it's that much harder.

Any fool who dares to criticise the pregnant Prime Minister as distracted, overworked or falling short will likely provoke hordes to rise up in her defence: women who've been through pregnancy, women who want to work through pregnancy and anyone who respects her right to have a child.

Actually, don't limit her invincibility to just the pregnancy period. It could go on as long as the child still looks like it wears nappies.

It's a bit strange to consider the political ramifications of something as personal as a pregnancy, but they are real. Nowadays, everything that happens in a politician's life is political.

And as far as politics goes, the Prime Minister's played this pregnancy perfectly.

Announcing something as joyous as a baby is a great way to start the year. It puts to bed the mini-shambles the coalition government found itself in at the end of its first few months in office, including a disappointing poll result showing no bump in popularity since the vote.

This is a reset. It makes the Prime Minister even more endearing by humanising her. It gives us something positive to talk about.

Then there's the slick move of giving Australian Prime Minister Malcolm Turnbull an early heads up. Once he heard the secret, he apparently called for a gushing chat. That's quite the turnaround from the guy whose government was quite obviously highly peeved off after Ardern publicly embarrassed it over the Manus Island refugee situation.

Still, the pregnancy news is not without its political risks. Anecdotally, some voters see it as selfish, irresponsible and cynical given that Ardern knew before concluding coalition negotiations with New Zealand First.

But, that's a low risk. Reason? Baby photos.

Once those arrive, the hard hearts of many cynics will melt. New Zealand will have its own version of the collective cluckiness Britain experiences every time Kate Middleton has a baby.

There'll be the baby's first photos. Then there'll be the baby walking. The baby's first words. The baby's first visit to mum at the office. Dad snapped at the playground on parenting duty. World leaders giving gifts of baby clothes. Magazines. Facebook videos. Instagram posts.

The baby will reach peak cuteness just in time for the 2020 election. It'll be ready for the first day at school just in time for the 2023 election.

And the political consequences of this pregnancy stretch way beyond. Young Kiwi women will surely benefit from the fact that the most powerful woman in the country is planning to stop what she's doing, have a baby, and then juggle a newborn and a crazy busy job.

The decision about when to have a baby is one many working women have to make. Ardern's announcement sets a new tone. If she can make this brave call, then no other women should feel she's letting her employer down if she too takes a break to have a baby.

For that move alone, Ardern deserves a huge round of applause.

It's hard to see a political downside to the Prime Minister falling pregnant. There are just a lot of benefits. For her and for young women now and in the future.

• From Tuesday, Heather du Plessis-Allan will be hosting Wellington Mornings on Newstalk ZB.