Which parent’s last name to give baby?

Definitely mums name.
If it works out and they do get married they can re-register the child as a child of marriage. Legitimacy laws still apply. The child gets a new birth certificate and it will show a maiden name for mum, regardless of whether she changes her name or not.
The hard facts are: the vast majority of parents split up. Unmarried parents are far more likely to split up than married parents. Young parents are even more likely to split up. This figure is not going to improve anytime soon.
So realistically if the child has dads name, it will grow up with a parent ( mum) who has a different name to them.
Add to this the very real likelihood that mum gets pregnant to someone else and that child has dads name so not only does the first child have a completely different name to their mother, they have to live with a sibling who has a different name and probably a man who has a different name. If the mum married her new boyfriend then everyone apart from the first child all have the same name. Just think how upsetting that is.
Of course there is the deed poll option, BUT father gas to agree to the name change. Plus why make your child have to constantly explain why they have changed their name?
Also, many second relationships fail too. So the poor child then as an adult changes their name AGAIN to either their real fathers name, or back to mothers maiden name!

HER name, without question. We’re in 2026 now, babies don’t automatically get the father’s name.
It’s so incredibly likely that they’ll split up at some point in future if they’re only 18 and she won’t want a different surname than her child.

What nobody has mentioned is that if she gives the baby the dad's name then she would have to get his permission later on if she ever wanted to change it. If she gives it her name, if they were to marry in future (and she wanted to change) then it's much easier to change both hers and the baby's at that point.

My firstborn is now 40 and when she was born unmarried women had just started giving their baby the dad's name (where we lived), so the practice is relatively new in the grand scheme of things.

Whilst others are saying that he will still have PR for taking the child out of the country etc, it’s much easier and you are less likely to be asked if your child has the same name as you. In fact I wonder, where the father isn’t in the birth certificate, how you prove this - I suppose you always travel with passport and birth certificate?

I think that all children should be given either only their mothers name or double barrel with mothers name last so it doesn’t get dropped. I’m very surprised that in 2026 young women are still changing their names.

Why assume that she will change her name if they get married?

Stay open minded, steer clear of assumptions.

Your DD needs to decide.

Depends, if the dad isn't going to be involved then hers, if she's planning to marry/take his name then it might help the dad feel included more to use his, depending if she likes that name. A lot of people hate double-barrelled names (and unless the names are going to be very long, when 2 people with such names marry, half of each will be dropped anyway), but if she likes that idea, then that would be a compromise.

Hers.

If only for practical aspects like getting a passport, travelling etc.

I was a similar age when I had my DC. DC has my surname, I would never have considered any other surname.

I'd encourage your DD to make sure her last name features somewhere within the baby's name. Spanish friends have used the mother's last name (or the first of her last names) and the father's last name, not hyphenated, for their children e.g. mum = Lola Cortez Garcia, dad = Tom Smith, baby = Isabel Cortez Smith

I imagine most people would automatically give the dads name, but she should give her own (for many reasons already listed), or at the very least double barrel

Hers.

DP and I have never married and our dd has my surname.

I've only come across the idea that there's pressure for children to be given the dad's name on Mumsnet.
I'm not sure if that's because it's a new phenomenon (my dd is in her early 20s) or if it's been around for a while and I wasn't aware of it.

It never crossed my mind that dd wouldn't have my surname. There are all the practical reasons mentioned already but I probably just absorbed earlier conventions because of all the 18th century and Victorian novels I had read.

Hers, as they are only 18 there will be a good chance they may not stay together, they can always add his name to double barrel it at a later date or change it if they are to get married in the future.

It's a relatively recent thing that the children of unmarried couples get the father's name.

The main thing is to make sure she’s aware that it’s him being on the birth certificate that means she needs his permission to take the child abroad, have a say in schooling etc.

A scary number of people seem to think that’s based on surname, and when it comes to PR it doesn’t matter if they use her surname, his surname, a combination or an entirely random name. The birth cert is what does that.

I would hyphenate both surnames. It’s his child as well as hers. The child will get used to whatever surname they’re given at birth, and it’s much more unlikely they’ll want to take their father’s surname later on if your DD and her partner remain together. People get used to the names they have, and don’t want to change them.

I hyphenated my son’s name, but made the strategic decision to put my surname first. I find quite often that when there is not enough room on forms or sports t-shirts that the second surname just naturally gets dropped . . . sadly. The full name can be a bit cumbersome, so Ive told him he’s welcome to use just the first surname if he wants, and drop the second. My DH has told him to drop the first part, and use the second 🙂. It’s DS’s choice ultimately though.

If she does decide to give the baby his name, make sure her surname is part of the child’s name. My DC’s both have my exH surname (I never took his name) but I’m not a fan of double barrelling so they both have my surname as a middle name.

Its up to her and her boyfriend. Your opinion or anyone else’s wont matter. Its really old fashioned to think the baby will have the dads last name

Id say hers or both, because of she does end up on her own.. but she might not feel that way. Is it a serious relationship?

The hospital defaulting to the mother’s name is to make the link between mother and baby obvious to stop any mix ups between babies (particularly when babies used to be taken off to the nursery).

One thing that has surprised me since doing genealogy is discovering that babies having their father’s surnames even when the parents were unmarried isn’t actually new. If the father was named on the birth cert the baby tended to carry his name. It’s just that very often unmarried mothers registered babies solo (and indeed in a lot of cases were wrongly informed they had to register solo) as they couldn’t name the father unless he was also there to register.

I agree with you - I have had several friends now who’ve used the dad’s surname and when I’ve asked why they’ve said “oh well Boyfriend is very traditional”… I always feel like saying, but resist the urge, that if he was that traditional, you’d have his surname too! 🤨

In terms of things being automatic, obviously yes you can register whatever you like. But in the hospital, they will default to the mother’s surname - I assume because that’s always been the way of things, and unmarried women using the father’s surname is quite a recent thing?

thanks for that information. I didn’t know babies could be reregistered so I’ll let her know that information.

It’s not quite that easy because if they aren’t together anymore (and he’s on the birth certificate) you can’t just change the birth certificate and re register. That is for when the parents were not married and later get married.

If she puts his name, she can’t simply reregister or change the name later.

Traditionally babies take their mother's name which often happens to be the same as the fathers if they are married