I am in a determinedly neutral room on Harley Street, my feet in stirrups, my dignity nearly preserved by a flimsy sheet. On the monitor in front of me is my womb, empty of baby but nevertheless being energetically narrated by Prof Nick Macklon. He points out my contraceptive device, my uterine lining gearing up for evacuation in about a fortnight, the visible vacancy left by my just-departed egg – Macklon describes it all as being like a pivotal scene in an action movie. The stakes, certainly, feel high.

I am midway through my “fertility MOT”: yes, like for a vehicle, but for my ovaries. The two-part process – offered by the London Women’s Clinic (LWC) and other fertility clinics like it, for a few hundred pounds – consists of a blood test followed two weeks later by a “not overly invasive” pelvic ultrasound. Alongside a consultation about your medical history and lifestyle, the results help you to understand your own fertility status and likely ability to conceive, now and in the future. “I must say, I really hate that term, ‘MOT’,” Macklon, medical director of LWC, had remarked earlier, back at his desk. “Women aren’t cars.” We aren’t cars – but there are a lot of fiddly parts, which we are mostly ignorant of until they don’t work.

If you had asked me half an hour ago – as I sat in the clinic’s stately waiting room, a baby’s cry faintly, ominously audible from afar – I would have put my grasp on my fertility as slightly above average. I knew, for example, that I had been born with a finite number of eggs and that, at 28, I was approaching the age at which I should be starting to take it seriously. But with the reality of children still entirely, ludicrously at odds with my day-to-day life, I had felt content to leave the picture blurry.

I wasn’t even sure that I wanted them, after all. From when I was a little girl to my mid-20s, I had steadfastly maintained that I would never have children of my own. In recent years, my flatmate had amassed a collection of photographs of me holding babies, their faces creased in terror, that, as a whole, seemed to form some damning body of evidence of my lack of maternal instincts.

But as I reached my late 20s, something started to shift, tediously on schedule. Choices such as whether to move country, or change job, or dump or not-dump a boyfriend started to seem properly consequential. Friends my age started admitting to concerns about “running out of time”; older friends who had witnessed too many heartbreaking struggles to conceive, urged me to take them seriously. And the reality of being a woman means you are afforded the luxury of ambivalence only for so long.

Nick Macklon: “Maybe it’s a good thing to plan these things”. Photograph: Sarah Lee/The Guardian

The MOT service, offered by LWC for the past two years, was born of two things, says Macklon: greater clarity about the impact of time on fertility, and the widespread lack of awareness of it. A 2018 meta-study concluded that “knowledge is insufficient, particularly in determining when female fertility markedly decreases” – and this despite the tendency to delay children until later in life. In 2016, the average age of women at first birth was 28.8 years, compared with 27.3 years in 2006. “There is clearly a lack of understanding among women that, essentially, their fertility ends about 10 years before they get their menopause, rather than when they get their menopause,” says Macklon. “That is the huge information gap.”

Though they may be getting periods, many women do not even start reliably ovulating every month until they are in their mid-20s. Fertility peaks soon after, then starts to decline, though “not catastrophically”, Macklon adds. “It’s probably a steady progression downwards from about the age of 27 or 28.” My age, then. I am starting to feel a bit panicked. The fertile window is from the mid-20s to the mid-30s – not exactly languorous, even with physiology on your side, and that can vary significantly.

I was correct that women were born with all the eggs they will ever have, about 1m of them, but that number has usually halved by puberty – and I am shocked to learn that we lose about 30 to 40 eggs each month. Before each period, Macklon explains with enthusiasm, “a whole group of eggs will have decided: ‘It’s our time now. We’ve been waiting 20-odd years; this is it, off we go.’” Hooked on hormones, they start growing.

Two weeks before ovulation, between about five and 20 eggs – a clutch, if you will – will be contenders. “Of those 20, maybe one will be a little bit more mature, a little bit more sensitive to the hormones” – and that frontrunner will start producing another hormone to stymie its competitors. “It’s very interesting sibling behaviour,” says Macklon, almost fondly. (And just as cut-throat as sperms’ race to fertilise, I note – without any of the mythologising.)

Elle Hunt undergoes fertility tests at the London Women’s Clinic. Photograph: Sarah Lee/The Guardian

Over time, women not only lose eggs, but their quality depletes. “The consequences are, as women leave childbearing until their late 30s, their chances of producing an egg that is of high enough quality to produce a healthy baby falls,” says Macklon. The fertility MOT can give an indication of “where an individual is on their biological clock”, and that there is demand for it reflects a more proactive approach to family planning.

Ranging in price from about £200 for a consultation with no pelvic ultrasound, up to £450 for a male and female package (and £640 for two women), the service is not cheap, and about half the 800 women a year who undergo the consultation with LWC do so as a first step towards starting fertility treatment, “rather than for reassurance or help with future planning”, says Macklon.

Those women whose MOTs return problematic results may also be advised to consider egg-freezing, prompting concerns that the tests could be steering them down an uncertain, expensive path.

The British Fertility Society (BFS) says that it is important to remember that these tests were developed to inform IVF treatment, not natural fertility, and as such are open to interpretation: “There is no doubt that tests showing a good ovarian reserve are reassuring but they by no means guarantee a baby; equally, a poor or impaired ovarian reserve does not mean you will struggle and need fertility treatment.

Dr Raj Mathur, the BFS secretary, says his view of egg-freezing is “cautiously favourable … but like everything, we are in danger of overselling its benefits and not adequately reflecting the problems”.

Similarly, he said, though the MOT might help some women and couples in their family planning, it should not be mistaken for a comprehensive diagnosis of individual fertility. “They are not looking at other aspects of women’s fertility – whether you ovulate or not, whether your fallopian tubes are blocked – and they are often not looking at the man. They are a kind of incomplete snapshot of one aspect of a couple’s ability to conceive, and that can easily give false reassurance.”

In 2016, the average age of women at first birth was 28.8 years, compared with 27.3 years in 2006

The blood-test component of the MOT measures levels of the anti-Müllerian hormone (AMH) given off by growing follicles, which indicates how many eggs you have left – but a study last year found that a diminished ovarian reserve was not associated with infertility. “If you’re just starting to try for a baby, a low AMH or a high AMH will not affect your chance of conceiving,” says Mathur. Macklon agrees that a single measure of AMH says little, but adds that in the context of the MOT, and discussed in the broader context of the individual’s medical, personal and lifestyle history, it helps to estimate the length of the patient’s reproductive period.

At the start of his career, Macklon says, he was typically tasked with problems of infertility: “You know: ‘I’ve tried for children for three years. It hasn’t worked; can you help?’ Now, it’s completely different.” Over the past decade, and particularly the past five years, he has been more often approached by single women and new couples wanting to know if they should get a move on. There is “increasing realisation”, reflected in the rising interest in the MOT, “that maybe it’s a good thing to plan these things”.

Yet many women – even those who know for sure that they want children one day – still leave any further interrogation of the idea until it is either a practical reality or there is possible cause for concern. “One of the hardest parts of our job is having to break bad news to women who don’t think they’re going to have a problem,” said Ben Cordle, a spokesman for LWC, when I called him a few weeks ago to make an appointment. It was possible for a fit, healthy woman in her early 30s with no reason to second-guess her fertility to record an AMH level as low as 2, from a healthy given range of 4.1 to 58.

In fact, the thought occurred to me only this morning: that this appointment I had made on no greater impulse than mild curiosity could change the course of my life. On my way to the clinic, questions that I had so far kept comfortably peripheral were brought to the front of mind, where they jostled for prominence.

The big one, obviously: what if I’m infertile? Then the shades of nuance: if I’m infertile, would I still write a story about it? What if I’m certified so fertile, I can safely push motherhood out to 40-plus – would I leave it as late as I can, or keep pace with my peers? Could I afford to freeze my eggs? Do I even want a baby? And the most frightening thought of all: what if the test says I must have one right now, or never?

In this tastefully carpeted clinic, returning to Macklon’s desk after my ultrasound, I am about to find out.

Relative to his somewhat whimsical tour of my womb, Macklon is all business as he goes over my results and medical history. I have a normal uterus. A normal endometrial thickness. My right ovary appeared normal, with 10 follicles – also normal – and “the left ovary looked similar”. My AMH level was “within the normal range for a woman of your age”, though on the lower side of the mean value, meaning that I might not like to leave having a baby until my late 30s – but I’d assumed as much.

The worst thing I could do between now and then, as far as my fertility is concerned, Macklon says, is take up smoking or get chlamydia.

It all somehow feels a bit anticlimactic. But then I realise I am lucky to have that result, lucky to know that I’ve got a few years to go yet. The MOT, Macklon says, presents “some options which we didn’t have in the past other than to say ‘get a move on’ or ‘you can relax’.” A remark he makes in passing lingers with me long after the appointment: “If you think about it, women have to get almost all their life events into a period of about 15 years: career, having children.” It is not long, but I have time – and just the process of finding out has focused my mind on how I want to spend it.