The dilemma: I recently had a short fling with a man I am very fond of. He ended it, which upset me more than I thought it would, and then I discovered I was pregnant. My close friends have been incredibly supportive, but I sense their disapproval. I am a professional woman in my mid-thirties and have been feeling the increased pull to have a family for a year, but I imagined doing that with a good man. My ex has made it clear he does not want me to go through with the pregnancy. But I am overjoyed to be pregnant, and am ready to be a mother on my own, though I'm not kidding myself that this would be easy or in the best interest of the child. On the other hand, I am realistic that the chances of me meeting Mr Right and having a family before I'm too old are getting smaller by the year. Am I being courageous or incredibly selfish? If I go ahead I know it will cause my ex a lot of anxiety, but this is not revenge for his ending the relationship or an attempt to get back together with him. By way of some background, he has just come through an acrimonious divorce, in which access to his young children was restricted.

Courageous or incredibly selfish, you ask. I'd say neither. It's funny, isn't it, that the background you choose to give me is about your ex. Having established quite firmly in your letter that this desire to become a mother is unrelated to him, you then decide to fill me in on his family circumstances. What about your own? Your ex has made his situation clear: he's reluctant to go ahead with having a baby because he did not want to continue the relationship with you. Are you guilty of trying to add Freudian dimensions to his understandable decision? The only need for you to add that final detail was to make even clearer why the poor man has reservations about becoming a dad again.

I say 'poor man', but the truth is I don't feel that sorry for him. Whether it's a one-night stand or a six-year relationship, it's disingenuous for anyone having sex to be surprised if their partner becomes pregnant unless they've been actively deceived. I'm seriously hoping that is not the case here. However, this does offer you some leeway. I'm not taking sides, but for every man who professes horror that his girlfriend is with child, there's a man who never considered prevention his business. If responsibility for his own actions wasn't a priority for him before, then this is, at best, a wake-up call.

Not that this exonerates you. I've no idea how culpable you are, but we are all perfectly capable of telling lies to ourselves. Are you quite sure this isn't something you helped along in an effort to secure an affair you didn't want to see end? A child born out of deception and a misguided determination to underpin a relationship is not to be recommended.

My instinct is to give you the benefit of the doubt. But while kids do emerge from much less functional unions than yours and live to tell the tale, there are medical arguments that back up your sense of urgency, and we ignore them at our peril. Both my kids were born after I'd hit 40, but it's not a route I'd recommend. No one need remind me how close I came to missing the boat. So while your fears aren't entirely rational as you hit your mid-thirties, and your panic is at present premature, your concern is justified. It's all too easy for five years to slip by and a dramatic decrease in your fertility to become terminal.

I realise none of this directly answers your question about what to do, but it's not a decision anyone can make for you. If you feel lonely and confused now, be aware that by embracing single motherhood those are emotional states that will become more familiar. There will also be rewards, but those you don't need me to outline. At present it's more important that you consider the disadvantages. Having a child isn't easy or idyllic, and however hard you imagine it will be it's 10 times more challenging. You'll most likely encounter discrimination at work, and it will restrict your career options at some stage. Do you have family or friends around you who will help shoulder the enormous responsibility and time-consuming drudgery of having a child? If not, the challenge you are setting yourself is huge and daunting.

I love my own children more than anything I imagined myself capable of feeling, but given the choice between not having them or having them alone, I'm not sure what my decision would or should have been. There are plenty of men and women out there who have chosen or been forced into single parenthood and are rising daily to the challenges. In this instance they're probably the best people to ask.

· If you, too, have a dilemma, email mariella.frostrup@observer.co.uk