Gestation and lactation. Those are the only things a mother can do for a baby that fathers cannot. But even breastfeeding needn't be an excuse for men to be excluded - or excuse themselves - from the picture.
From this weekend, a number of NHS Trusts across the UK are marking National Breast Feeding Week.
It is worth us all remembering that mums are more likely to pursue and succeed with breastfeeding if the father is actively involved right from day one, and that baby-father bonds are stronger when dad plays a role.
As I show in my book about shared parenting, Men Can Do It!, the path to unequal parenting often begins at breastfeeding, simply because on a physiological level it can be an activity that excludes men.
It then strengthens the relationship between mother and baby, separating the father from the action and often giving him a reason to excuse himself.
A 2011 survey revealed two in five fathers don't get up in the night, and a third sleep apart from the mother in the first year to get a good night's sleep.
But how can dad get and keep involved when he doesn't have... the equipment?
I may be pro-breastfeeding, but this article isn't about its benefits or otherwise. Instead, it's about how to keep dads involved if couples do decide to give the breast milk their best shot.
As someone who was involved with breastfeeding right from the start with both of our children, here are my top tips to help fathers get involved.
Treat breastfeeding as a joint decision
Don't regard breastfeeding as something which the mother decides whether to do or not, with fathers only in a supporting role. Or worse, as if it has nothing to do with him. Discuss the pros and cons together before the baby is even born, then after the birth remember it was a joint commitment you made to each other, as well as your new baby.
Start early
Almost half of fathers said they were "pretty well ignored by maternity services", according to The Fatherhood Institute. Don't allow this to happen. On the maternity ward, as well as in antenatal classes, insist that the midwives and experts show fathers how breastfeeding works too, and the different techniques you can try.
Share the night feeds
Night feeding can be one of the most stressful and exhausting part of the whole new baby experience. My wife and I took a one-night on / one-night off approach. On my nights I'd fetch the crying baby, bring her to my sleeping wife and help the baby latch on to my wife's breast. I'd then sit awake until the baby had finished feeding, burp and change her nappy then put her back to bed. It took us some time to get it right, and you'll want to work out your own way. But that doesn't mean dad sleeping in another room because he has work tomorrow.
Get to know the icky bits
Bleeding nipples, spilled milk, burping, vomiting, nipple guards, hard lumpy boobs and baby poo can all part of breastfeeding. They'll be much easier to deal with if both mum and dad get to know the ups and the downs, and work out ways to deal with them together. Dads can rub in nipple cream too, and mums don't automatically get first dibs on those mustard-filled nappies.
Share the expressing
Expressing isn't for everyone, but it does give the father a hands-on role bottle feeding the baby breast milk during the day and the night. That helps build the bond between dad and baby, gives mum her turn to rest, and allows a fairer opportunity for women to go back to work. Try to share the expressing by each parent taking turns to wash up afterwards, sterilise the bottles and suction cups, label the bottles, and to pack and carry the not unsubstantial equipment if you go away.
Think strategically
If you're in a cafe or other public place, consider together how to make the mother feel more comfortable - physically and emotionally - for breastfeeding. Can the father sit somewhere to lessen the mother's public exposure? If you're planning a long journey, plan where will regular stops might be made. A lay-by on the A14 probably isn't the most comfortable place for any of you to get the breastfeeding job done (though, as I can attest, it has been done).
Don't be too quick to give up
It's awful to see both your partner and your baby in distress, so there is a temptation for fathers to urge abandonment if breastfeeding isn't going to plan. But sometimes the mum - or the dad - wants to keep trying. Keep talking about it, respect each other's point of view, and keep looking for solutions together before jointly deciding throwing in the towel.
Support each other as joint participants
Dads can offer practical support to their breastfeeding partner: fetching cushions, glasses of water, muslin squares and something for mum to read. But remember: men are part of the breastfeeding experience, not just supporters and helpers. Dads can sit with their partners while they feed, help the baby to latch on, stroke the baby's head, look into its eyes and hold its hands or feet. Mums should remember to allow the father 'in', not bark instructions and roll their eyes if he makes mistakes. After all, women are just as likely to 'get it wrong' as men are.
Dads can ask for help too
Most Primary Care Trusts and many parenting charities run breastfeeding support groups. There's no reason why dads can't call to make appointments, ask for advice, and even attend breastfeeding sessions as well as mums. Some, but certainly not all, groups are rightly women only. But many group leaders will spare five or ten minutes at the end to see mums and dads together, in private if necessary.
Good luck!
Gideon Burrows is the author of Men Can Do It! The real reason dads don't do childcare, available now.