Kamin Mohammadi, who is in her late 30s, is single and childless. So would the arrival of her oldest friend's first baby change their relationship or even signal its death knell?

A year ago this week, I was waiting for a call that I looked forward to and dreaded in equal measure. I had agreed to be the birth partner of Justine, my oldest friend, and though we had discussed everything from her birth plan to who to call in the event of an emergency, it was only after she passed her due date and started eyeing up the castor oil that the birth finally became an unavoidable fact. Quite apart from trepidation about the event itself, I started to contemplate nervously what would happen when Justine was not just my great friend, but also a mother and the head of her own little family.

I was nervous because although I regard the arrival of every baby born to a friend with joy, I also wonder if it will sound the death knell for our friendship. I have made it to my late 30s without having children, and I am becoming a rarity in my group of friends. Since reaching 30, when the first of my close friends gave birth, the parenthood trickle has turned into a flood and now many of those friends are on to their second or third children. While for me that has meant a sizeable collection of delightful godchildren and endless fun, it has also brought the loss of several friendships I had held very dear.

Inevitably, relationships will change when one friend becomes a parent and the other stays childless. With one in five women childless by the age of 40, this is a situation many of us face, from either side of the fence. According to the Office for National Statistics, the number of 40-year-old women without children is twice as high as it was 20 years ago, a trend that will continue.

Close friendships provide us all with a sense of being cared for and contextualised, and this is especially true for those of us seeing out our adulthood as single people. Today, where family set-ups can be fractured, it is usually with our friends that we have our longest emotionally intimate relationships.

When friends have babies, the gap of experience that opens up can be hard to bridge. The new family unit, with its exclusive "gang of three" membership, provides a vital cocoon for the baby but it can also present an insurmountable wall to outsiders, one that sometimes I find myself unable - or unwilling - to scale. And babies will bring out the cracks in all relationships, not just those between father and mother. Friendships I had previously regarded as solid have sometimes turned out to be less durable.

The joys of parenthood are hard to share and for the gap in experience that suddenly opens up, the new parents might as well have taken a trip to Mars. I have watched dear friends turn from intelligent, engaged people into gurgling, cooing aliens and they in turn regard me with a distant gaze as if from an emotional galaxy far far away. Suddenly, evenings together are cut short and yawns are stifled on both sides.

How the parent responds to this can be a key factor in determining the shape the friendship will take post-baby, and I am afraid it is very much the parent that sets the tone - after all, they are the ones whose lives, interests and priorities have suddenly changed. Some of my friends have tried to convert me to the club while others have made valiant attempts to stop themselves from evangelising. My friend Richard Flint, who has a daughter, has weighed up carefully how to strike this delicate balance: "Do you keep away because you don't want to bore them with tales of nappies and sleepless nights? They might be offended if you do; it might seem that you think they can't deal with it. So you have to inflict just enough of the parenthood stuff on them to make them feel included, but not so much that you can no longer relate to each other."

I can see that caring for children occupies so much of the time and thoughts of parents that it can be challenging to find other things to talk about and I don't wish to alienate my friends who may be feeling isolated already. Another friend, Christobel Kent, who has five children, recalls: "I spent the first 10 years of my life as a parent very cut off from old friends for fear of boring them, and it is easy to retreat for good."

In my own experience, I have loved every single baby that has been born to a dear friend and have found them, by and large, fascinating. However, while I am willing to listen to the finer details of little Isabella's pooing habits or Joshua's colic, I would like the same courtesy to be extended to my concerns. The problems begin when I can see that my parent friends cannot be bothered to get their sleep-deprived head around the issues I am grappling with while I am expected to imaginatively enter a world that is alien to me.

For some this never becomes an issue - as long as the childless friend does not mind limiting the ensuing friendship. Robert Jacobs, a childless documentary-maker, has never felt the need to integrate with his friends' family groups: "If the friendship is solid, it will survive regardless," he says.

In fact, he is honest about something many of us childless people dare not speak about with our parent friends, our guilty secret: "Watching them makes me glad I don't have children - they soak up every part of your life and while I can appreciate life becomes totally enjoyable in a different way, it's not in a way I find appealing. My friends respect that and I respect them."

But in my experience, it's not all that easy and the close friendships that have survived babies have taken work on both sides. One such is with Clare Naylor, one of my oldest friends who became a mother last year. She thinks it's all about honesty. "It has been OK for us because I am prepared to be honest about motherhood in a way that not many new mums seem to be. I love being a mother but I haven't forgotten that I had a very different life before that, which some days I miss."

As a childless woman, I have often resented the implication from parents that my life is not somehow quite as valuable as theirs now they have discovered the Greatest Love of All and are engaged in doing The Most Important Job in the World. This is not something that is ever vocalised but is apparent in many ways, the most extreme being a friend who would not accept the possibility of my being left happily childless and promised to come after me with a turkey baster and sperm if I turn 40 in my present condition. Christobel admits that, as a parent: "One does become rather evangelical. The whole thing about how it is absolutely possible for a woman to be happy without children suddenly seems completely unbelievable and you go through a phase of feeling it is your urgent duty to communicate this message."

Richard echoes this sentiment but recalls his own feelings when a friend, on finding out he was going to become a father, confessed to him that she and her husband "secretly feel really sorry for people who don't have kids".

"I remember thinking - well that's all very well now I am about to join your club," he says, "but what if something was to go wrong? What if, after all, we are left childless? What will happen to our friendship then?"

As an empathetic - if childless - woman, I can appreciate this zeal and even tolerate it, but ultimately it did drive me away from my turkey baster-wielding friend. Christobel acknowledges the futility of trying to convert friends: "Some people will stop talking to you altogether. And there is no recorded instance of any woman acquaintance of mine having a baby because of how great I told her it was so, in addition, it doesn't work."

Nor, necessarily, does the attempt to draw the childless woman into the family, if it is not matched by an equal attempt by the parent friend to occasionally exit the family too. One particular friend was so keen to include me in her family circle that I was expected to have dates not only with her and the kids, but also alone with the children and with the family as a whole - and all that before we even got to the two of us spending time alone together. I found my already busy schedule being booked up months ahead by the complex series of relationships I was having with just this one family alone. Exhausted, I let the friendship slip with some relief.

Sometimes it feels as if battle lines are drawn based on our fertility, yet another arena in which we compete to prove the validity of our life choices. But emotional maturity and great understanding can see us through. My former teenage partner-in-crime Beth Ley-Greaves, now a mother of two, said recently: "I don't want to call you and complain that I am covered in sick and the husband is late home from the pub again and I am fed up, and maybe you won't call me and say, 'I am home alone on a Friday night, I can't face another bad date and I'm lonely.' We make false assumptions about each other's lives when all we really need to do is communicate."

True friendships that survive are based on change and adaptability to new situations. Since Justine gave birth to Rudy a year ago, we have endeavoured to keep sight of each other's different concerns, as well as sharing our love for Rudy and, as a result, have formed our own little flexible family. Justine says, "It can be quite an effort not to disappear into your own world and block out other people. But in making that effort, all my relationships have improved."

And as Christobel points out, ultimately love wins out: "Generally the friends who properly love you - including, in my case, a friend who couldn't have babies and for whom the subject is still painful - and can find those other things to talk about ... a surprising number of them ride out the whole thing with you."