Signing up for NCT antenatal classes ranks alongside buying a Bugaboo pram and joining Mumsnet for many middle-class pregnant women.

New figures show that there has been a boom in the number of women paying around £200 to attend a course of private NCT sessions – a jump from 25,000 in 2005/6 to 40,000 in 2010/11 – while the provision of free NHS classes in austerity Britain is increasingly patchy.

Despite a discount scheme from the NCT for the poorest families, a two-tier antenatal system is emerging, where less well-off women have little access to the advice and support so crucial for first-time mothers. Along with the scrapping of the health in pregnancy grant and child trust funds and changes to child tax credits and the Sure Start maternity grant, it is yet another area where mothers on low incomes are losing out under the coalition's cuts programme.

Yet, it also raises questions over whether private classes run by a charity whose core philosophy is the belief in natural childbirth best prepare women for giving birth.

Having experienced both NCT and NHS classes, I found the latter were more objective. The midwives running the NHS course, held in a soulless room at the GPs' surgery, provided a practical guide to late pregnancy and birth, covering labour symptoms, the various types of pain relief on offer and the different positions which could facilitate delivery. In contrast, the NCT classes were held in the teacher's home where far more subjective advice was served up along with cups of tea and biscuits.

For some, the letters NCT conjure up images of teachers proselytising about drug-free labour to a group of parents who favour cloth nappies and think formula milk is akin to feeding your baby neat gin.

My friend Jane recalls how her teacher told her group at its first meeting that she had fried up her placenta with garlic and onions and served it to her family for supper the night she got back from hospital with her third child. At a later class she told them to avoid taking the pain killer Pethidine during labour, which is offered by some hospitals, because it could make their children drug addicts in later life. When asked what evidence she had for this she said she'd read it on the internet. Another friend, Michael, who went along to classes with his wife, remembers the "week of the cloth tit". To demonstrate how to breastfeed, their teacher produced a knitted woollen breast from her bag and a sock puppet baby.

These are extreme examples, and pain relief, Caesarians and bottle feeding are all up for discussion at NCT classes. Even so, some feel there is still too much emphasis on having a natural birth. Many pregnant women go to their first class thinking they'll have an epidural and by the end of the sessions they have convinced themselves that labour is not simply about having a baby but is something to "experience" and even enjoy. In the worlds of many NCT teachers, women should control the birth, resisting drugs and medical intervention by breathing deeply, perhaps with the help of a Tens machine.

The reality is that for many women complications set in or the intensity of the contractions is just too painful and they end up hooked to oxytocin drips, anaesthetised with an epidural, with their feet in stirrups and surrounded by doctors with forceps at the ready. The experience is frightening because it is not the calm, soft-focused delivery they had expected and afterwards they feel like failures for not having had a natural birth.

Of course, there are hundreds of thousands of women who are zealous in their enthusiasm for the NCT, saying it made them feel empowered going in to labour and – this is the main reason many join – helped them establish a mums' network. Motherhood can be an isolating experience because families and friends are often scattered across the country. Meeting women who live locally and have babies the same age can be a life-line during maternity leave and beyond.

There is more to the NCT than its antenatal classes – it is also a powerful lobby group. The organisation was founded more than half a century ago with the aim of winning women the right to a natural childbirth at a time when giving birth was medicalised and doctors knew best. Over the years it has campaigned for fathers to be allowed into the delivery room, for shaving and enemas – common practice until the mid 80s – to be dropped, and for labour rooms to be more comfortable.

Given that, according to a survey by the parenting website Netmums, 30% of first-time parents are not offered any NHS antenatal care, the NCT do fill an important gap for those who can afford it. Just remember you don't have to buy completely into its whole philosophy.