There is something about being at the cutting edge of medical ignorance that takes the sting out of male infertility. You are not alone, and the scientific community is falling over itself trying to work out what is wrong. The latest evidence to confirm the trend came from the Aberdeen Fertility Centre this week, which reported a 29% drop in sperm counts among men attending the centre between 1989 and 2002.

This looks bad compared with the previous generally accepted figure of a 50% drop over the past 50 years worldwide, although the Aberdeen statistics could well be skewed by the fact that these men and their partners were seeking fertility treatment. Nevertheless, the research also confirms the other worrying part of the fertility problem: nobody knows why it is happening.

All this confusion and gaps in the knowledge come as no surprise to me. Having been diagnosed infertile two years ago, then subsequently becoming the father of twin boys through IVF, I was unprepared to hear my wife say, late last summer, "Darling, I think I'm pregnant."

We stared in disbelief at the blue dot on the home pregnancy kit. Suddenly I understood why they sell the things in packets of two. We re-read the instructions a third time, and used the second tester. The blue dot persisted. This wasn't possible. I had the fertility test results and the earnest, embarrassed chat with my GP, to prove it.

After a year of trying for a baby without success, we had decided to have ourselves checked out. I was the one with the problem. I had a low sperm count of one to two million per millilitre, compared to a normal range of 20-200 million (the Aberdeen study reported a drop from 87 million to 62 million over its 14-year period of analysis). What sperm there were had "low motility" (under 50%) and "high abnormality" (a shocking 1% of my sperm were normal). In other words, they were swimming slowly around in circles, whimpering softly to themselves. No crack Olympic team, these.

I can't remember what the doctor said when I asked about our chances of conceiving naturally, but the gist was clear: a fraction of a fraction. There was no point in us just "carrying on trying", as most couples in their late 20s would be advised. But we were lucky to have found out when we did, as we still stood a good chance of success with IVF. Female fertility is of the essence, it seems - though my wife's test results were fine, her scores would start decreasingly badly in her mid-30s.

Psychologically, it wasn't too much of a blow. I had never really been much of a macho type, never made it into the first football team at school, had lost the few half-hearted fights I had ever got into. It was in the abstract that it was harder to take: that Mother Nature had decided to do without my genes in her secret plans for the future of the human race. Later, when one friend congratulated us on the good news of our pregnancy with the jokey remark, "We'd all been worried you were firing blanks," I took a twisted pleasure in telling him he was right and watching him squirm.

And it is easy to talk about being infertile when you have two gorgeous twin boys bouncing on your knees. It becomes part and parcel of the whole smug-dad routine, and a lot easier than dissembling or lying when people ask (as they always do) "Do you have twins in the family, then?"

Still, the reasons for male infertility remain unclear. If my problem isn't genetic - and my parents had no problems conceiving me or my sister - then what is the cause? I'm not overweight, don't smoke, drink excessively or wear too-tight trousers (and when was that last a problem for men of my generation?) - all of which have been posited as possible causes. Or could it be something beyond my control: too-tight nappies as a baby? Trace elements of pesticides in the water? Or even of oestrogen, thanks to the contraceptive pill? Now, that would be an irony.

Which is where we came in, staring in disbelief at the plastic tongue of the pregnancy kit. In some ways, this is far from a rare occurrence. Getting pregnant boosts female fertility in the time after giving birth. The drugs involved in IVF boost it further. Nurses, doctors and health visitors constantly remind new parents about contraception. Mostly this earns a weak grin to match the dark-ringed, bloodshot eyes. Who would think about having sex at a time like this? Look where it gets you.

But when doctors talk about the boosted fertility of female IVF patients as a caution against new parents going at it like rabbits, they are only really talking to the 70% of couples with undiagnosed fertility problems - couples who do fine in the tests, but just don't seem to have come up with the goods. In our case, they knew what the problem was, and boosted female fertility was, frankly, neither here nor there.

It seems that our lucky break was just that, a proof that very long odds - a fraction of a fraction - might put an end to rational hope but they don't rule anything out. As we prepare for life with three children under 20 months, few things seem certain any more. Just one does - once this third baby arrives, I will only be allowed near the marital bed if I'm carrying an authenticated certificate of successful vasectomy. From firing blanks to facing the snip, all in two years: who says parenthood isn't a wild ride?