Lizzie Harrop is hoping she might receive a box of chocolates or a handmade card on Sunday, but what she is most looking forward is spending lots of time outdoors in the Leicestershire countryside with her three-year-old son, Barnaby. Today is another Mother’s Day she can mark as a mum – something she once thought would happen. At 39, she became pregnant with Barnaby unexpectedly after failing to conceive several years.

“We given up trying because the strain was taking its toll on us. I stopped taking drugs that made me ovulate and was trying to accept that it might not happen. And then I got pregnant. I’m still pinching myself.”

A hundred miles north in Wakefield, West Yorkshire, Lisa Bucknall, also 43, is watching comedies on Netflix, eating chocolate and locking herself from the world today. “I dread Mother’s Day. I always loved children and thought one day I’d be a mum. But I didn’t meet anyone I wanted to settle down until I was 35.”

She and her new partner started trying a baby right away. “We thought it would be really easy. It came as a shock that it wasn’t.” Three IVF attempts, with both her own and a donor’s eggs, ended in failure, miscarriage and separation. “It feels something has been snatched away from me.”

The different experiences of Harrop and Bucknall will strike a chord for many women in their late 30s and early 40s on Sunday. Many – those who are celebrating Mother’s Day with a newborn for the first time – are mothers because they had options their own mothers didn’t have. But at the same time the rate of childlessness among British women over 45 has rocketed a level not seen since the aftermath of the first world war.

About one in five (18%) of British women 45 are childless, the Office for National Statistics revealed last year, and Britain has one of the world’s highest rates of childlessness among women aged 40-44.

According to a global survey by the Organisation for Economic Co-operation and Development (OECD), UK women aged 15-39 said they would personally like to have 2.3 children on . Asked what they thought the ideal family size was generally, just 0.67% of UK women surveyed said zero children.

Fewer women today start a family in their 20s – the average age for a British woman to give for the first time now is 30, compared with 26 in 1971, and the ONS reports 54% of women are over 30 when they have a baby. That figure is even higher for men becoming fathers: 68%.

A whole host of financial reasons  behind this shift. As well as being the most debt-laden generation and the first expected to pay significant fees for university, 25-to-34-year-olds are far less to own their own home than in the past, with almost half living in rented accommodation, compared with almost a third only 20 years ago. While the average age of a first-time buyer in the early 1970s was 27, today it is 33. This often impossible battle to secure enough money to buy a home of their own means a quarter of millennials aged 20 to 34 still live with their parents.

Changes in women’s working lives have also had an impact on men’s attitudes to family life, argues social gerontologist Dr Robin Hadley. “Some men today feel more ambivalence about when to settle .” Often, he says, this is not because men don’t want children, but because they want to be able to play more of a nurturing in their children’s lives than their own fathers did. In 1974, men were twice as likely to have a child in their 20s as their modern counterparts, who also get married almost 10 years later.

Women who have children when they are younger often struggle to cope financially. Childcare costs in England have risen up to seven times faster than wages since 2008 and it now costs £6,300 on average for a part-time nursery place for a child under two, with a quarter of mothers under 25 reporting they had left a job because they could not care.

“Often, women over 40 are childless because they have made sensible, honourable decisions,” says Jody Day, 53, founder of the Gateway Womensupport group and author of Living the Life Unexpected, a self-help book for childless women. “We used birth control so we didn’t get pregnant accident, we held out for the right partner and we waited we had a secure home environment to bring our children in. We thought we had plenty of time in our 30s because of IVF. But then it became a race against the clock – and time ran .”

This rings true for Bucknall, who says she took it for granted she would be a mother one day, but never had unprotected sex before the age of 35. “I left school thinking you could get pregnant at the of a hat. Taking precautions was ingrained in me.”

In her opinion, the disconnection between the number of children that women under 40 want to have and the number of children that women over 40 actually end up having is due to misunderstandings about fertility. She believes this is partly caused messages about how easy it is to get pregnant that were drummed into her generation at school, along with widespread myths about IVF.

Earlier this month, she helped the newly established Fertility Education Initiative lobby the government to start educating children about fertility as part of the national curriculum: “We’re focusing on 16-to-18-year-olds, but age-appropriate conversations with children about fertility should be happening from year dot. The viability of a woman’s eggs as she ages and the science of IVF is not well understood by the young. We should be celebrating what IVF can do and how it can create modern families for women with fertility issues, but also acknowledging limits.”

Infertility is not, of course, the only cause of involuntary childlessness. It is estimated that the vast majority (80%) of childless older women are childless to circumstances, rather than choice or infertility.

Even for women who are happy they are childfree, Mothering Sunday can be frustrating because of the inference that achieving the status of mother deserves a special day in the calendar each year.

“Possibly the thing you can do on Mother’s Day is to presume that every woman is a mother, or wants to be a mother, or will be a mother one day,” says Day. “Another painful aspect of the day for many of us is the idea that we are not kind or nurturing we are childless. This strikes at the very core of feminine identity. Today, childless women can feel both intensely visible – because we don’t ‘fit’ – and invisible, because where is the space for our grief?”

This an experience recognised by Bucknall. “I’m the only one in my circle of friends who doesn’t have kids. You can feel like something’s wrong with you, like you’re not a complete woman.”

Julia Bernard-Thompson, 40, a business consultant from Ashford, Kent, is one of the many women for childlessness has been an active choice.

“It first hit me that I didn’t want kids when I was 13. Later, I would always tell potential partners as soon as the topic came up, even if it was the first date,” she says. “I met my husband when I was 29, 11 years ago. When I told him I felt about having children, he audibly exhaled and said he felt the same way, and it was so rare for him to meet a woman who didn’t want kids. That was the moment we knew we were going to be together.” She says the couple enjoy their freedom and independence, and spending long, lazy days together. They also like being able to spend their money themselves.