The shout rang out as I lumbered inelegantly across the packed school hall. "Oh. My. God. Surely it must be about to come out?" This was from another mother. And me with seven weeks to go until my due date. The shame of it. Later in the playground came another gleeful cry and a barely suppressed giggle. "Waddle, waddle!" I blushed. I tried to walk faster. (I found I could not.)

The next day in a department store, an assistant simply looked at me and burst out laughing. "If I were you, I wouldn't turn around too suddenly, madam," he winked. "Our displays are quite fragile." Oh, how we laughed. And how I longed to be lactating so I could have squirted him squarely in the eye.

Now the due date – early August – is getting close, the comments are ever more frequent and more personal. From friends and strangers alike, they are accompanied by wide-eyed expressions of horror and hilarity. "Blimey. Not long to go, then … " "Still here?" "Sure you haven't got two in there?" "Oh, my goodness. I don't what to say. You're so … large!" And my favourite, at 34 weeks, from a twentysomething man with, I presume, scant biological knowledge: "Wow. If it's that big now and you've still got six weeks to go … How is it going to come out?" How indeed.

What is it about heavily pregnant women? As a species, there are plenty of us about. In terms of physical presence, there are certainly plenty of us to notice. In the modern world we are unapologetic about the space we occupy, and rightly so. Many women nowadays work right up to within a few weeks or even days of the birth. So it's not as if we're invisible in the workplace or in public. Yet we are still, strangely, somehow an oddity. The more pregnant you get, the more people stare and pass comment, as if in the presence of a giant, lumbering freak. They often look frightened or intimidated. Either that or they veer towards the other end of the spectrum, sweetly and politely insisting that "You're not that big," "You look very compact," or "That's a neat bump you've got there."

Having already had two children – Will, six, and Vera, four – I feel I should be able to shrug off being an object of universal ridicule during these last, unwieldy weeks. But you never quite get used to it. The ritual humiliation always starts around the 30-week mark for me. To be fair, I'm not a skinny person at the best of times, and by seven months pregnant I do look as if I could give birth at any moment. So I understand people's bemused reaction, though far less so with other mothers who must remember what it feels like to know you're the biggest person in the room. You try to laugh it off. But when the fourth person that day cracks the twins joke, it is not easy.

It helps that my children at least seem very at ease with my growing size. Earlier on, before I looked properly pregnant, they were worried. Vera once pointed out an obese woman in a cafe and asked loudly, "Mummy, are you going to get as big as that?" But now they don't see a big tummy or even a bump: they see a space filled with a baby. Friends their age are similarly relaxed, if they notice I am pregnant at all. Children tend merely to register that there is a baby in there, shrug and move on. What's the big deal?

Adults just cannot do the same. They are the complete opposite. They have to say something. They can't not comment. And most people – especially people who don't know you at all – feel they can speak to you in a way they would never dare speak to anyone else. Is it to do with discomfort over weight gain? Probably. After all, pregnancy is the only condition that allows you relative safety to tell someone they've bulked up without being cruel or rude. But there's also Schadenfreude in there too. ("Thank God she's that large and I'm not.") Some people really do seem to relish being able to tell you how big you've got. Needless to say, this does not commend them to me.

The only thing worse than people unnerved by a heavily pregnant woman is people who are convinced they can detect the gender of your baby by scrutinising the bump. I have never found out the sex of the baby in any of my pregnancies: I would rather have a surprise. One woman, a complete stranger, stared at me for several minutes, eavesdropping on a conversation I was having ("No, I don't know what the sex is … ") and then screamed in my face as if I was stupid, "You're having a boy." At such moments I must physically restrain myself from shouting, "Oh, so you're God, are you? This is how you know? Or you just like making bets where there are only two possible outcomes and so you have a 50-50 chance of being right?"

I similarly require sedation if I come under heavy questioning about when the baby is coming out. Guess what? I don't know because I do not have prior divine knowledge of all life on earth and I'm not having an elective caesarean. Both my previous babies were over a week late. So who knows? If I'm 38 weeks pregnant, it could be any minute. Or it could be in almost a month. You can't say that, though, because then people start looking terrified. A whole month? Or right now? Yes! Imagine the sense of frustration and anxiety if it were inside you!

It doesn't help that you have to contend with all this at a time when you're at your most emotionally vulnerable. If only the gigantic pregnant belly could either be treated a little more nonchalantly or with more tacit respect. Because the closer the unknown date gets, the more you have to steel yourself to leave the house and run the gauntlet of these well-meaning but often infuriating comments. Which is a strange state of affairs when we're supposed to be relaxed about bumps these days, not expecting women to retreat into "confinement" as they did in days gone by. So much so that sometimes I wonder if we've moved on at all.

If you look at maternity dress patterns from the 1950s and 1960s, the models are barely showing. It was seen as indecent and undignified to be seen in public while heavily pregnant. Presumably, a lot of women would just stay at home when they reached a certain size. Even in the 1980s, when the parenting expert Heidi Murkoff, author of the What to Expect When You're Expecting series, was pregnant with her first child, she felt she could not display her "condition" too publicly: "I can remember wearing a one-piece bathing suit when I was pregnant with my first baby – instead of a maternity suit – and people looked at me as if it was obscene."

That can't be the case now, can it? It's almost 20 years since Demi Moore posed for the cover of Vanity Fair, one arm around her breasts, the other cupping her pregnant belly. The issue sold out even though some newsagents refused to carry it and others covered it, like X-rated porn, with a brown paper bag. "After showing Demi Moore's huge belly, why not on your next cover have Bruce Willis with a huge erection?" one reader wrote to the letters' page.

Interestingly, Moore was only seven months pregnant at the time. It would have been more daring, perhaps, for her to be photographed more heavily pregnant. Anything over about 32 weeks is a shape we still rarely see portrayed in the media, which is perhaps why it still paralyses people up close. The feminist academic Carol A Stabile writes that the pregnant body – even clothed – has the ability to "shock and horrify the spectator". She continues: "Not only is it perhaps the most visible and physical mark of sexual difference, it is also the sign for deeply embedded fears and anxieties about femininity and the female reproductive system."

People feel weird thinking about other people having sex. That is why pregnant women are scary and funny. Someone had sex with that woman. And look what happened.

It helps to dwell on these ingrained reactions when you're feeling stung by yet another person sniggering "good-naturedly" that you're the size of a house. I'm finally finding that the uncomfortable emotions and socially inept reactions that pregnancy elicits can actually, perversely, make you feel closer to other people. In the last few weeks I have started to wonder if there is something more fascinatingly unconscious going on when people comment and stare. I can almost read strangers' thoughts as they blink fleetingly, nervously at my belly, rarely making eye contact with the actual host vessel (me). You can see them gulping slowly and wondering, "Is that where I once was?" "How is it going to get out?" Those looks can feel intrusive, but they're also quite touching in a way. They remind me of something a friend once told me. That the best thing you can do if you're feeling scared of giving birth is to walk into a crowded shopping centre, take in the throng of people and think: look at them: they were all born. It happens all the time. It will happen again.

Similarly, I've been trying to allow myself to observe other people's discomfort with the bump rather than get upset by it. And slowly the reactions of others are becoming less annoying and more liberating. You can see people connecting the dots and working it out. "Yes, that is where we all were once. How weird that we can't remember it. How strange and wonderful."

It struck me recently that people might say some inane things – but in the main they can't help themselves and they mean well. And all they really want is exactly the same thing as me. Dear God, let it come out safely. And please let it come out soon.