Published: 15:54 EST, 1 October 2014 | Updated: 07:34 EST, 2 October 2014
Sitting in the waiting room of the antenatal clinic I was aghast at the sight of the women around me. To my left, a lady awaiting a 20-week scan munched her way through an enormous bag of crisps. It was barely 10am.
Another woman had spread a napkin over her expansive thighs and was devouring an oversized 'breakfast muffin'. These, as any self-respecting woman knows, are just calorie-laden cakes masquerading as a healthy meal. This is a ruse that only ever seems to fool the overweight.
All around me I spotted arms, chins, thighs and bottoms. Dimpled, corpulent flesh was everywhere. If it weren't for the pregnancy bumps, this easily could be mistaken for a slimming club meeting. Only at a slimming club, there is a sense of shame, or responsibility and desire to change.
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No excess weight gain: Sadie at three months pregnant, left, and two months later at five months pregnant, right, she doesn't look much different
'When are you due?' a woman asked me. 'Four weeks,' I replied proudly. Her eyes dropped immediately to my compact bump, and I could see her mental processes working overtime.
Her initial incredulity rapidly morphed into anger at me and pity for my unborn child. 'Anorexic,' I could tell she was thinking. 'Obsessive. Narcissistic. Putting her own vanity before the health of her baby.'
In a word, she was seized by jealousy - the green-eyed monster that seems to turn the most sensible and intelligent of women into bile-spitting harpies when faced with something they wish they had.
I suspect that was the real reason behind 75-year-old feminist Germaine Greer's attack on the Duchess of Cambridge this week. Her argument was that Kate, who happens to be suffering again from chronic morning sickness, was 'too thin', and that she 'shouldn't have been made to go through all this again so soon [after the birth of Prince George]'.
Her outburst infuriated me, as I too came in for constant criticism just because I chose not to put on mountains of blubber during my own pregnancy, which culminated a year ago in the birth of my bouncy, happy, healthy son, Albie.
Despite the bleatings of all those who attacked me for refusing to eat for two, we were both declared to be in perfect health just hours after the birth.
Why won't people wake up and acknowledge that the real risk to unborn babies comes when their mothers are obese?
Staying healthy: Sadie had a small bump at six months, left, causing her friends and even strangers to criticise her for not eating enough. Kate also had a neat bump when she was pregnant with Prince George at six months, right
Dr David Haslam, chairman of the National Obesity Forum, recently warned: 'Obesity in pregnant women can lead to all sorts of problems, including the death of the mother, or of the baby through stillbirth, or the baby having foetal abnormalities, or the woman suffering pre-eclampsia or gestational diabetes, or requiring a caesarean because either she or the baby is too big.'
I firmly believe that if more mothers adopted my - and Kate's - approach to pregnancy, scant NHS resources would not be wasted treating the problems associated with maternal obesity. Every woman has a duty to exercise and eat healthily before and during pregnancy - for her own good and the good of her unborn child.
'Are you eating properly?' one old friend demanded when I met her for dinner at seven months. 'Isn't it time you let go of your need to be slim for once?' snarled another
The morning of that antenatal appointment for example, I'd already done an hour's workout in the gym and eaten a bowl of organic porridge for breakfast.
This was all part of my campaign to ensure I only gained minimal weight during pregnancy. And, I'm pleased to say, it worked. Four weeks before having my baby boy by caesarean section - essential due to complications with the placenta - I was still wearing my pre-pregnancy size 10 tops. I never bought any maternity wear at all - except for some pregnancy jeans with stretchy waistbands.
My pregnancy coincided with Kate's first and I would examine photos of her forensically. I felt absolutely inspired by her mini-bump and otherwise slim frame.
Of course, the mummy-tummy mafia weren't so keen, branding her selfish for remaining trim and claiming this put pressure on ordinary mums not to pile on the pounds.
For me, Kate - known for her love of sport - was welcome proof that not every mother-to-be has to turn into an overweight, out-of-shape slob. And like Kate, I was seven months pregnant before my bump even started to show in my clothes.
Starting to bloom: A more noticeable bump at seven months, right, and seven-and-a-half-months, left, but she was still slim compared to the other expectant women she met who had seen pregnancy as an excuse to pile on the pounds
According to my midwife, it was the legacy of my tight abdominal muscles, developed through years of regular exercise. She estimated I'd gained just a stone-and-a-half when I was full term.
I was well within the healthy weight-gain boundaries.
Experts recommend you should put on no more than 21 to 28lb while expecting, a combination of baby, placenta, amniotic fluid, extra blood and around half a stone of fat laid down naturally in preparation for breastfeeding.
But, just like poor Kate, I had to defend myself throughout, to everyone from friends to other pregnant women and even strangers.
I'm not ashamed to admit that I was also terrified of losing my figure out of vanity. What's wrong with that? Why would I suddenly want to be fat just because I was pregnant?
At five months, when I still had no visible baby bump, friends started to voice their concern, telling me sanctimoniously that I was failing my unborn baby. This went on right up until the moment I gave birth last September. 'Are you eating properly?' one old friend demanded when I met her for dinner at seven months. 'Isn't it time you let go of your need to be slim for once?' snarled another.
One close girlfriend was particularly vociferous, exclaiming: 'You've not put on nearly enough weight, you need to concentrate on nourishing your baby.'
That hurt, as it insinuated that I wasn't putting my son's health first when that couldn't have been further from the truth. In the gym there were looks of disdain on the faces of other members at the sight of a pregnant woman lifting weights.
And out powerwalking - I stopped running outdoors at 17 weeks - the disapproving tuts of passers-by were audible.
When I wrote in this newspaper last year about exercising till I was eight months pregnant, I was vilified, with critics even suggesting my child would be born underweight or with shaken baby syndrome. Yet when did you ever hear anyone berate a pregnant woman for putting her feet up, gorging herself on cream cakes and chips and whacking on a pile of unnecessary weight?
All I did was continue to exercise daily and eat a diet rich in chicken, turkey and fish, lots of fresh vegetables and homemade soups.
Slimming World's WOTY on post-baby weight loss (related)
Full-term: Sadie had gained just a stone-and-a-half - and she was back in her size 10 jeans just over a fortnight after giving birth
Proud: Despite the mummy mafia's reservations, Albie was born weighing a very healthy 7lb 6oz
All of this was done with the blessing of my obstetrician, who said that she wished more women would take the approach I had.
My priority was to be the healthiest I could be for the good of my baby. But I'm not ashamed to admit that I was also terrified of losing my figure out of vanity.
What's wrong with that? Why would I suddenly want to be fat just because I was pregnant? My husband's only concern was that I and our baby were healthy. But I've always taken pride in my appearance and would have seen it as disrespectful to him to let myself go.
All the research backs up my sensible approach, even though the mummy mafia won't agree. Countless studies have proved that a sensible diet and being active throughout pregnancy are good for the woman and her unborn child.
The Royal College of Midwives' policy adviser, Janet Fyle, is unequivocal regarding nutrition and the dangerous 'eating for two' myth. 'In the womb, babies take what they need in terms of nutrition, leaving the mother with the remainder. Therefore eating more than the baby requires results in excessive weight gain and that will be the mother's problem in the end.'
Weighing in: Germaine Greer has said the Duchess, pictured left shortly before it was announced she's pregnant again, is too thin
In fact, no extra calories are needed until the final three months of a pregnancy, and even then only a further 200 a day are recommended, the equivalent of one extra slice of toast or a small bowl of cereal.
My argument with the 'thin haters' was well and truly won when my beautiful son Albie was born weighing a very healthy 7lb 6oz.
And just 16 days later, I was back in my size 10 pre-pregnancy jeans and gleefully stuffing those maternity skinnies into a cupboard.
Unfortunately, the jealousy didn't end there - as it didn't with Kate, whose body recovered very quickly after the birth of Prince George too.
One friend told me, with a distinct sniff, that I would doubtless gain weight over the coming months as I sat around feeding and cuddling my baby. I didn't.
Would I find it more comforting to see Kate porking out and straining at the seams of her maternity wear during her second pregnancy? Absolutely not. I shall be watching, and admiring, her all the way.