Some women are desperate to have children - but some are desperate not to. Here two who want to stay child-free explain why. Holly Brockwell, 29, from London, has been trying to get sterilised, while in Tehran thirty-something Nina Nikoo (not her real name) faces family pressure to get pregnant.
As a woman, there are four little words I can say that invite more condescension than almost any others: "I don't want children."
"But why?" people ask, as if there's a simple answer to why you viscerally, instinctively reject something that's considered a fundamental part of humanhood.
The fact is, there's nothing about creating another human that appeals to me. That's an emotional thing, and translating it into rational reasons takes something away from its strength.
If I say I don't think I'd be a good parent, for instance, people respond, "Everyone feels that way at first." If I say I can't imagine ever having the time, energy or money, I'm told I'll "find a way to manage". If I say I want to devote my life to my career, they say I'm "selfish".
There's no acceptable reason to not want a baby, it seems.
You'd think, from the responses people give, that everyone who procreates is ecstatically happy with their choice.
I know categorically that this isn't true, because it happened to my mum.
She's never hidden the fact that she didn't want kids in the first place, and only agreed to have them because my dad was desperate for a family.
It's partly my own fear of capitulating that's driven me to try getting my tubes tied - to take the choice away in case I'm ever tempted to betray my beliefs for love. After being told four separate times that I was "too young to even consider it", despite the fact that there's no minimum age for sterilisation in the UK, I finally got referred this year.
I was ecstatic - until I tried to arrange the operation. Marie Stopes, who do the procedure for the National Health Service, told me matter-of-factly that there were no surgeons available, and I'd have to go back to my GP. In the meantime, I'd moved into the area of a different NHS Trust - which has meant starting the whole process again.
You may wonder why I don't choose another, less drastic, form of contraception but the pill has been making me sick for years and the only other option is the coil, which I'm not willing to have because I know two people who've experienced horrendous side-effects.
I don't need reversible contraception. There's a 10-minute keyhole operation that can solve this problem for good, and I can't believe that at the age of almost 30 in 2015, I'm still having to fight to get it.
We can choose to get pregnant at 16 but not to decline motherhood at 29. It seems our decisions are only taken seriously when they align with tradition.
Well, I've never been one for tradition. I recently started a tech website written by women - I'm proud to say it's the only baby I'll ever have.
Holly Brockwell will be taking part in a Q&A on the 100 Women Facebook page on Tuesday 24 November 15:00-16:00 GMT. Please visit the page to leave your questions.
I think I'm very lucky to be a woman, but unlike many, I have never felt maternal. I have always thought it is a crime to bring a child you don't want into this world.
I've worked very hard to set up my own business. I now employ six people and find nothing more satisfying than my job. Some people think I'm selfish, I don't know, perhaps I am. But regardless of what people think, I can't give up on a dream that after so many years has recently come true.
My parents were shocked when they heard that I don't want to have a child. They still bring it up at every chance they find. And it's not just them. Other family members try to convince me that I'm making a mistake. I remember in the first years after my wedding, which was about 10 years ago, people were very judgmental. They even suggested that I or my husband were infertile and that we were hiding it.
They have now more or less given up but my parents are very persistent. My father says one day my biology will make me want to have a child. The other day, my mum was combing my hair and said it made her very sad that I will never get to experience what she has experienced. Like my dad, she thinks I will change my mind.
- Levels of childlessness tend to be highest in parts of Northern/Southern Europe and East Asia, and lowest in Eastern Europe and parts of Southern Europe and Central and Western Asia (UN)
- Childlessness appears to be related to educational attainment - in Switzerland about 21% of all women age 40 are childless, but this rises to about 40% for women who have completed tertiary education (OECD)
I think the fact that many of my friends are childless, even though they have been married for many years, helps a lot. Having a child is a burden for educated women in Iran. It means you can't concentrate on your job, your freedom is limited and if your marriage doesn't work out, your chance of finding another husband is low.
Don't get me wrong, I love children. I am patient and can easily get down to their level and spend hours playing with them - just as long as they aren't mine. When I see a child hanging off her mum's neck, I feel suffocated. I'm so happy that she's not mine.
From day one, I told my husband that I didn't want a child, and he seems OK with it. I can sometimes see in the way he looks at children that he wouldn't mind being a father but he respects my decision. Convincing his parents was difficult too.
I do think about it every day, though. In fact, I wish I could find motherly feelings in myself. I do wait for the day that I might change, however unlikely that seems.
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