Roy Kerrigan thought his school must have been built on an ancient Celtic fertility site. Or perhaps there was something in the water. What else could explain seven members of staff all falling pregnant at the same time?

“There was a running joke that you had to be careful where you sat,” says Roy, head of Two Moors Primary School in Tiverton, Devon. “People said certain seats were responsible. It was wonderful that so many teachers had babies, but seven is pushing it a bit.”

However, convincing data, alongside basic human biology, suggests there is nothing mystical going on. In fact, as the demographics of the profession shift, it’s a wonder there weren’t more babies born to teachers at Two Moors School.

That is because teachers are getting younger, according to the General Teaching Council for England (GTCE). There are 11,000 fewer teachers aged 45 to 59 than there were a year ago and 10,000 more teachers under the age of 45. And those younger teachers are likely to be women: there are 63,500 female teachers in their 30s across England, an increase of 12 per cent since 1998, the Department for Children Schools and Families (DCSF) confirms. If women between 25 and 29 are included, that figure rises to 112,000 - a third of all teachers.

Now consider that the average age of married first-time mothers rose above 30 for the first time last year (or 28 when unmarried women are included), as the Office for National Statistics points out, and a problem starts to emerge.

“We’ve got a large number of female teachers in their 30s, which is the key childbearing age,” says John Howson, a recruitment analyst and visiting professor of education at Oxford Brookes University. He points to the 1,083 secondary maternity cover posts advertised in England and Wales during the past school year, out of a total of more than 200,000 full-time equivalent secondary teachers.

“It could have a significant effect on the labour market if about a quarter of the workforce is off on maternity leave. There will be more temporary jobs, and it’ll be increasingly hard for schools to plan for, especially inner-city schools that have a large proportion of younger teachers.”

Roy Kerrigan saw his school’s pregnancy boom as a challenge rather than a problem, but admits it was a logistical nightmare trying to fill all the short-term vacancies.

“We’re lucky that we’re a big primary school with 500 pupils, so although the system creaked, it didn’t break,” he says. “Our large team of staff - we’ve got 30 teachers and 30 teaching assistants - managed to roll with the punches, but a smaller school would have been stuffed. It would have been too expensive to get all that supply in.”

Last year, six of the seven pregnant teachers, who are all aged 28 to 30, came back to Two Moors School on a part-time basis. But others never return. Nationally just one third of women who leave to have children come back to the classroom, an analysis by the DCSF in 2006 suggests. It is not just an issue for the female-dominated primary sector. Secondary teaching is also undergoing a seismic change as young women step into the shoes of middle-aged male teachers approaching retirement, according to research by Manchester University.

The report, commissioned by the NASUWT teachers’ union in March 2008, warned that women could soon be as numerous in secondary schools as they are in primaries, where they make up 84 per cent of the teaching staff. Three-quarters of new teachers are women and they are slowly replacing the 32 per cent of male secondary teachers who are aged more than 50 and nearing retirement.

“We are moving from a largely experienced workforce, made up of over 45- year-olds, to an inexperienced younger female workforce who may want to take a substantial time off to have children,” says Professor Howson. “It could prove problematic.”

Those who return to the classroom can face discrimination. For instance, they often lose additional responsibility payments when they come back from maternity leave, the Manchester University report found.

Anastasia de Waal, head of family and education at the think-tank Civitas, believes the stress of the job can also deter returnees. “Women don’t see teaching as a lifelong career anymore,” she says. “They stay for shorter periods because they feel they’re reaching absolute burn out, especially when they’re having to juggle the extra pressures of motherhood.”

Alison McClean, a history teacher, is a case in point. Of the 15 women who have had babies over the past couple of years at Backwell Comprehensive School near Bristol, she is one of just two who have returned to full-time teaching. The single mother of two finds it hard balancing childcare with teaching, but says she can not afford to go part-time.

“I get up at six in the morning and rarely sit down again until after eight in the evening,” she says. Despite having an extremely supportive senior management team, extra-curricular activities and parents’ evenings pile pressure on to an already gruelling timetable, which includes teaching 250 pupils.

“I’m not at all sure how long I will be able to continue working under these conditions,” says Alison. “Instead of expecting parents to take a pay cut in order to reduce their hours, more attention needs to be given to reducing the workload of all teachers.”

The economic crisis may reduce Alison’s options still further. She, and other female teachers in her position, may decide to stick with their current job rather than risk job insecurity in the private sector or at different schools. Other women may decide that maternity leave makes good financial sense in the current climate, argues Professor Howson. “This recession could incentivise women to take maternity leave, where they’re at least receiving some pay, as opposed to an unpaid career break,” he argues. “If they were to get pregnant again during their 52-week statutory maternity leave, they could be off school for years, with the obvious difficulties that places on school planners.”

Teachers who remain in the classroom are already seeing their workload increase as a result. Paul is in favour of his primary school supporting female teachers on maternity leave, but felt aggrieved when he was forced to cover an absent teacher’s curriculum areas and responsibilities without a pay rise.

“My colleague earned teaching and learning responsibility (TLR) payments, but I was told I had no chance of being rewarded financially for the work I took on,” he says. “When she returned she decided that she’d take her foot off the gas but would not relinquish the payment she received. I was left in the position of either watching a crucial area of the curriculum (English) flounder or continue with my added responsibilities unpaid.”

The effect on pupils’ performances is also a bug-bear. Continuity and strong personal relationships can be disrupted by any sort of leave, however good the cover.

“You can have a teacher taking a GCSE class from the start of Year 10 to the February of Year 11 and then go on maternity leave right when their experience of that class is perhaps most valuable in the run up to exams,” says a teacher who wishes to remain anonymous. “I don’t bemoan female colleagues’ rights, nor do I suggest that they should plan their pregnancies around the requirements of the exam season, but it’s undoubtedly an issue.”

Certain so-called “feminised” subjects are likely to be more affected than others. For example, 13 per cent of art and design posts advertised during the last academic year in The TES were to cover maternity leave vacancies, compared with just 4 per cent in the more male-dominated IT sector.

Different issues arise when members of the senior management fall pregnant - something that is becoming increasingly likely as younger teachers become leaders. In 2006, there were 35 secondary headteachers aged 30 to 34, and more than 530 in primaries, figures from the National College of School Leadership (NCSL) suggest.

“It’s unusual to run a school on a temporary basis, but that’s what’s happening as young heads and heads of department take maternity leave,” says Professor Howson.

About half of heads in England are over 50 and the NCSL estimates the number of those retiring will peak this year at 3,500, followed by a decline to 2,500 in 2016. As the “baby boomer” generation reaches retirement, and the age profile of teachers continues to downshift, schools will need to recruit teachers to headship earlier in their careers. But this necessary step will leave some older women “stuck in middle management for 25 odd years”, says Professor Howson.

Women who have children in their 30s may return to work five years later, perhaps in a TLR post or as head of department. But their career options will have become limited by then, Professor Howson warns. “People in their 40s have often already missed the boat in terms of headship, especially in secondary schools. A head of department can’t just become a headteacher, but they don’t necessarily have time to pick up the experience they need as assistant heads or deputy heads.”

His findings are based on the 12th annual survey of Education Data Surveys, a research firm headed by Professor Howson. “For someone starting (teaching) at 30, and then taking a career break or even maternity leave for a year, the timescale (for becoming a headteacher) starts to become more problematic,” it concluded.

Even if they are still young enough to aim for headship, schools do not always provide the sort of flexibility would-be leaders (and mothers) need. Jane Bailey, 35, gained her National Professional Qualification for Headship (NPQH) just before having her first child in 2003. She had her second child two years later. Despite wanting to apply for headships in North-west England, Jane’s role as deputy head began to take its toll, and she went part-time in 2007.

“I would love to take a headship role, but it would mean losing all the benefits of working part-time,” she says. “There must be so many women in this position. I can’t see why, with the current recruitment problems for headteachers, there is not more scope for co-headships and part-time headships. It must be the way forward when so many women are reaching the stage to apply for headships at the same stage as they have young families.”

The TES website has recently established a forum to help people like Jane find a job share partner in their area (see below for details). But not all schools are willing to offer part-time roles to new mothers, especially those in the senior management team, a poll for England’s GTC suggests.

The survey of 3,665 teachers found that female teachers were more likely to put their families before their careers. When they are denied job-share options, some choose to leave the profession altogether or move into supply teaching.

Up to 47 per cent of supply teachers (15,718) are aged between 50 and 64, the GTC’s annual statistics show. Keith Bartley, GTC’s chief executive, believes these older supply teachers could ease the headteacher recruitment crisis.

“Many teachers take on supply roles towards the latter part of their careers,” he says. “As the teaching profession gets younger, these very experienced teachers have an invaluable role to play in school.”

But schools would need to embrace creative solutions if they are to keep these valuable resources, argues Alma Harris, professor of educational leadership at the Institute of Education in London. “Schools in the future will need to have much greater flexibility around contracts and working practices,” she argues. “The fluidity of staffing, rather than being a problem, can be seen as an opportunity to bring in different expertise at different times.”

This can be achieved by factoring flexibility into any forward planning, but it will require support staff playing a much greater role, she adds.

John Bangs, from the National Union of Teachers, agrees that any potential shortages will not be solved on an individual basis, but by schools working together and potentially sharing staff as the need arises.

“More women going on maternity leave is an issue, but it’s utterly manageable through a bit of contingency planning,” he says. “The mechanics are already there in the form of school federations, partnerships and clusters. There needs to be a pool of teachers in the local authority or local area, who are familiar with the school and pupils and who can be called upon to fill any short-term posts.”

But contingency plans are unlikely to be formalised if the Government does not recognise there is a problem. “This is all speculation and conjecture,” a DCSF spokeswoman says. “Our teacher supply model already takes into account likely retirements and predicted maternity leave effects. The primary workforce has always been largely female so if Professor Howson’s assumptions (about maternity leave) were true we would have seen that effect already. Instead, recruitment to the primary sector remains healthy.”

Professor Howson takes these criticisms in his stride. He believes the DCSF teacher supply model is out-of-date and only uses partial information. “The DCSF fails to take account of the fact that a third of new teachers are aged over 30 when they start teaching,” he says. “So it’s not surprising that the department is seemingly unaware of changes happening under its nose.”

For those on the ground, a young, female-dominated profession is hard to ignore. Roy Kerrigan is delighted that most of his new mothers returned to Two Moors, not least because they are all fantastic, committed members of staff. But then he hesitates. “Having said that, I hope we won’t have that many taking maternity leave at the same time again.” Figures suggest that may be wishful thinking

Visit the TES job share forum

The maternity deal

If you fall pregnant

- Teachers must tell their employer 15 weeks before the baby is due.

- If you have been working for the school for 26 continuous weeks by the time you are five months pregnant, you are entitled to 52 weeks’ maternity leave including 39 weeks of statutory maternity pay (90 per cent of your average weekly earnings for the first six weeks), dropping to Pounds 117.18 per week for the remainder of the time.

- Employers are entitled to make “reasonable contact” with teachers on maternity leave. This should not include being asked to work from home.

- Teachers have 10 “keeping in touch” days, when they can come into school without losing or ending their maternity pay. Teachers should receive full pay for these days.

- Teachers who become pregnant again during their maternity leave have the right to further ordinary and additional maternity leave.

www.direct.gov.ukenParentsMoneyandworkentitlements.