Couples trying not to have a millennium baby next January 1st should exercise extreme caution this weekend, when what conception experts call the "optimal fertilisation window" is wide open.

In fact the window was open yesterday, too, and you had better hope you did not fall through it last night. Unless, of course, you're one of those deranged people who actually plans to be in a maternity ward on New Year's Eve, delivering - excuse me while I use another medical term - a "Y2K love bug".

There are enough such people already to fill the world's baby hospitals on the night, even as every piece of computer-based medical equipment on the planet crashes. It's the likes of these who have been splashing out on millennium conception kits: "fertility guide, ovulation pre diction test, pregnancy test, massage oil and hand-dipped candles" (answers on a postcard about the candles), as advertised on the Internet.

But since the chances of anybody successfully plotting to give birth on a particular day are only about 1 per cent, the vast majority of these lunatics will be safely at home on New Year's Eve, in spite of themselves. The likelihood is that ordinary, sane people like you or me (assuming one of us is female) will win the millennium baby lottery without even trying.

Which is why it is vital none of us buys a ticket this weekend.

A few simple precautions for the wary. For one thing, you should avoid watching ITV tonight, when the station presents programmes designed to put viewers in the mood for something other than turning over to BBC.

Also, you should on no account tune in this weekend to the British radio station, Classic FM, which will be broadcasting passionate music from the likes of Ravel and Rachmaninov, the effect of which will be to have listeners pulling the clothes off each other.

On a more basic level, you should stay off alcohol, of course. Although, since this is also the first piece of advice given to couples trying to get pregnant, maybe you had better decide that one for yourself.

In general, you can simply reverse the sort of tips being posted all over the Internet to those trying to create Y2K love bugs, like this one for women: "Lying still for at least five minutes after intercourse increases the odds that sperm will fertilise the egg."

So the advice is, if you do succumb to unplanned intercourse this weekend, get up immediately afterwards and shake yourself.

Afternoon is said to be the best time to conceive, but the plethora of televised sporting events this weekend should take care of that problem. If you are at a loose end, though, exercise will take your mind off other things.

A swim in Dublin's Forty Foot, for example, will help cool you off, if you're male; while if you're female, just looking at those fat blokes with no clothes on will probably do the same.

Be warned that even if you get through this weekend, you cannot afford to relax. The curtains of opportunity will not be quite drawn over the fertilisation window until about April 23rd, and even then the possibility that the first millennium baby could be a premature delivery means it might be advisable to stay celibate until July.

In the meantime, there are some signs that sanity is prevailing, even among the Y2K love-buggers.

Fans of the US Website WWW.babycenter.com may be familiar with the scary case of Duane and Susan Dimock of San Diego who, as Brian Boyd pointed out in these pages a couple of weeks ago, planned to get one or other of them pregnant around now and then beat the rest of the world by flying to the international date-line island of Kiribati next New Year and having a Caesarean birth at 00.01 a.m.

But the latest bulletin from the Dimocks quotes Duane as saying the trip is now unlikely, because his wife is afraid the birth would become a media spectacle. What an idea! "She's getting weird about it," says Duane (whose bright idea this was, you sense).

Meanwhile, for doctors and midwives who might facilitate this sort of thing, a word of caution.

There is a sobering precedent in the California doctor who, according to the San Francisco Examiner, timed a delivery for the first minute of January 1990 so that the child could be rushed to a nationally-televised broadcast. The paper adds: "State officials yanked his licence."

And whatever that involves, it sounds painful.