"I've got three sisters and two brothers, so I'm used to there being lots of people about. You just have to embrace it."
Conveniently, her mother, ex-Blue Peter presenter Janet Ellis, lives just 10 minutes away (she and Sophie's TV producer father, Robin Bextor, split up when she was 4).
"I rely on her pretty heavily. She's so smart and gives each of the kids their own special grandma time."
Eldest child Sonny is also a useful extra pair of hands. "He's my secret weapon. He's fantastic with Jesse. The boys are actually all obsessed with their baby brother."
Does she mind that her music commitments frequently take her away from them? "I hate the idea of them noticing, but I have to work - and I want to work. So it's really just logistics."
She and Richard, the bassist with band The Feeling, try to manage their workload so that one of them is always at home in west London. "He's a very calm, hands-on dad, and he's also great at scooping them up for one-on-one time."
One tabloid newspaper has claimed that Sophie is "addicted" to having babies, which she refutes. "I don't think I'd have used that word. I just always liked the idea of having lots of children, and I love the way the dynamic changes every time."
The title of her new album, Familia, hints at the importance of the "little people" in Sophie's life. "They're ... absolutely where my heart is. Jesse was in my arms in the recording studio, which gave the album the homely, relaxed feel that I wanted."
I've been a working mum with little people to think about since I was 25.
Earlier this northern summer, Sophie got social media abuzz when she revealed that her decision not to wear "ridiculous" stomach-sucking Spanx on a night out was thwarted by a woman asking if she was pregnant again. "It was horrible, so I wasn't sure whether to post anything about it. But then I thought, 'Sod it.' Yes, it does leave you with a bit of a tummy. So what?"
Sophie's attitude to body image stems from her own upbringing in London.
"My mum banned the word 'diet' in our house. She was such a good role model, and I've never believed in fads or weighing myself either. We like our food, but also like feeling good about ourselves. I genuinely enjoy the gym, and that feeling of being strong enough to run and jump on stage while I'm singing."
A trainer at her local gym has been helping her get back in shape since Jesse's birth. Having had to shift pregnancy weight four times now, she knows the drill well. "Of course I always want to feel like myself again, but ... it doesn't have to happen overnight."
Ellis Bextor and Richard first got together when they were both 24, and they discovered they were expecting Sonny after just a few weeks. He was born in 2004, and the couple married in Italy in the summer of 2005. But while they are regarded as having one of the strongest marriages in showbiz, there is apparently no magic formula.
"We just make an effort to spend time together, and we truly listen to each other. It's the small, everyday kindnesses, too. Simple things like making each other a cup of tea and sending little messages during the day are the glue."
Perhaps inevitably, her parents' divorce influenced her views on settling down. "When Richard asked me to marry him, I definitely wanted to, but I was determined that what happened with my mum and dad wouldn't happen to me."
In June, the pair celebrated their 11th wedding anniversary in muddy Glastonbury fields - "the first night we'd had away together since having Jesse" - but Sophie says that, for her, the festival was tinged with post-Brexit blues.
"It was awful. I had such a heavy heart. But you don't vote for change if you're happy, so we have to listen to why people are unhappy. We can't just dismiss it as ignorance, because not everyone is blessed with great opportunity."
Sophie says she's raising her children to be politically aware. "I don't approve of people who say they don't read the news because it's depressing. What gives anyone the right to turn their back on what's happening in the world? Now, more than ever, we've got to listen and try to help one other."
Her first two pregnancies had been difficult; Sophie developed pre-eclampsia - high blood pressure - and both Sonny and Kit had to be delivered early. Is she tempted to add another child to her brood, making it five, as Jamie and Jools Oliver recently did?
"I honestly don't know. If four is my lot then I'm happy. But then again ...
"I get so excited every time someone new comes into the family. It's just the loveliest feeling."
