Fathers! Attend the birth of your children. David Cameron's latest paternal call to arms, given this Tuesday in a speech to the National Family and Parenting Institute, is rousing stuff. Childbirth, the Tory leader said, is the "magic moment" that cements the bonds which hold families together. No dad should miss it.

Some mothers, of course, may beg to differ. Though around 96% of men now attend the birth (compared with only 5% in 1965), it is debatable how helpful their presence is for labouring women. Obstetrician Michel Odent, the "father" of waterbirth, has long maintained that birthing women are better off without their partners in the room. With their "logical" brains, and understandable anxiety, Odent argues, dads can seriously interfere with a woman's ability to use her "instinctive" brain to get the baby out.

Melanie Every of the Royal College of Midwives agrees that sometimes dads are just not that helpful during labour. The RCM recently conducted a study of fathers attending childbirth, which found men saying things such as "I don't know why you're making such a fuss, that contraction hardly registered on the print-out" and "How much longer is this going to go on? I'm getting really hungry."

As if this were not enough to impose a national ban on blokes at birth, many women in the RCM study also - not entirely surprisingly - felt inhibited by their partner's presence. Some even feared their partner would not fancy them any more if he witnessed unsavoury things such as poo, sick or anything gory at the "business end".

Since Cameron is presumably all for "informed choice" in childbirth, it is perhaps worth running through a few deleterious dad possibilities so all women can become fully informed.

Probably the most common male manifestation during labour is Saviour Dad. Many men find it disturbing to watch their beloved partner experience outrageous pain. The problem is that while women in labour frequently look like they are in hell, they are often actually coping brilliantly. For a loving partner - particularly the first time around - this distinction tends not to exist. Their beloved is suffering and they don't know what to do about it.

"Men will often want to save the woman," confirms Kim Kelley, a midwife for 25 years. "That's what they are conditioned to do. But women, in childbirth, often don't want to be saved." It is not unusual for the dad to be the one begging for an epidural, to stop his partner's pain. Since epidurals are associated with a significantly increased likelihood of medical interventions, even this tender impulse can backfire as Saviour Dad finds himself elbowed aside by medical personnel brandishing scary-looking implements.

At the other end of the spectrum is Sugar Dad - Sugar as in Alan. Sugar Dad has cortisol pumping through his veins. He is simply not prepared to sit back and do nothing: "He was shouting orders at me - literally," says Karen, who is married to Steven, a banker. "I practically expected him to shout 'you're fired!' at the midwife. I was trying to push, and he was yelling 'Push! Push!' next to my head. The midwife had to ask him, gently, to go and get me a drink of juice from down the hall. We laugh about it now, but at the time I could have murdered him."

One problem is that men generally like to be "doing" something when stressed. But once you've calculated and executed the best route to the hospital, unloaded the car and checked the camera batteries for the 19th time, concrete tasks beyond brow mopping and back rubbing tend to tail off. Some dads find this psychologically taxing.

Which could explain the strange, but relatively common phenomenon of Laptop Dad. "I have seen this a surprising number of times," says Julia Guderian, a doula (childbirth companion) for 15 years. "Dads in the hospital with their laptop buzzing - it can be their way of coping with what is otherwise just too stressful". Laptop Dad may also be found watching Sky Sport as labour cranks up, urgently needing to check his fantasy football team, or finishing that pesky spreadsheet as his wife dilates to 10cms.

A survey by the National Childbirth Trust found that a quarter of men felt frustrated during the birth that they were unable to help when their partner was in terrible pain. Such existential stress is probably in part responsible for Tech Dad who hones in on the technology instead of on his wife. He cranes over the bleeping monitors, trying to decipher the multicoloured heart rate print-outs.flashing lights and decode the foetal Basically, he is looking for proof that this whole thing is, really, under control.

And control is probably the key to the final puzzling phenomenon: Helmet Dad. Some men just need to be the centre of attention during labour. "I had one father who insisted on wearing a cycle helmet throughout the entire labour - in case he fainted," says Guderian, who knows such attention-seeking behaviour is not uncommon among nervous dads. "Virtually every person who entered that room was drawn to Helmet Dad. They commented and joked with him and he soaked up the attention as his partner struggled through painful contractions."

It is hardly surprising that some men find the emotions of labour overpowering. Childbirth has it all - life, death, fear, pain, agony and ecstasy. The vast majority of women say that having their partner with them during the birth was enormously helpful. And ultimately, says Every, "whether or not the dad is at the birth really has to be the woman's choice." I certainly would not have been without my husband during any of my three labours. But before it becomes mandatory it is worth pointing out that not all dads are cut out for labour. Men, in other words, need to put their helmets down, and let the women choose.

· Lucy Atkins is the author of First-Time Parent