First of all, let me assure you that nothing I have ever done has given me so much joy and delight. Having a baby to raise is magical: they are members of the family from day one, full of idiosyncrasy and intelligence, magical in their ability to apprehend the world around them for the first time and still really get it.
However, please don't assume that nothing will change. Here are a few honest truths.
1) Tell your partner you think you should see other people. Er, person. Small, demanding person that you will both love madly, derangedly, passionately. You will find yourself telling your child that s/he is your favorite person in the universe, right in front of your mate. You will kiss the bare skin of the baby a million times in the first month -- hundreds of times a day. You will dangle upon a toothless smile. This swoon of love is wonderful, but it will change your relationship with your partner; you will not have much time together, and if you do, you will speak in glowing tones about the new girl/guy in your life the whole time. Ouch. Look, this is truly nice, but you will find yourself getting misty about the times when you could spoon your lover's back and just listen to him/her breathe.
2) Your body will no longer be your own. Let alone the gross and magical things that happen to you while you are bringing the baby into the world, as I am sure you have thought about them -- and maybe you have come to terms with peeing yourself, etc. No. I am talking about all the other stuff. The hours spent expressing milk in one fashion or another. The long days without a shower or even sometimes brushing your teeth. Skin on skin, the baby's on yours, until you cannot stand another touch. Being poked, prodded, punched, slapped, pulled, tweaked, suckled, bitten, drooled on, sneezed on, vomited on. Having tiny handfuls of hair pulled out painfully or torn off. Not being able to wear your glasses without incurring a sharp, if clumsy, punch in the nose. Being used as furniture. Having your spider veins and moles picked at obsessively by an observant little ape with sharp fingers. Being kissed with a wide open and drooling maw. Getting slapped in the face with a drooly hand. Waiting to pee until you do actually pee yourself while bending down to change a diaper, all because you dare not disturb your progeny. Getting strong in the arms and back from lifting but flabby in the middle because you never go out anymore. Saying goodbye to sleep. Saying goodbye to your bed -- both for lovemaking (if you share a room with a bright-eyed light sleeper) and for sleep (since said light sleeper will often demand to cozy up with you, taking up the whole bed.)
3) You will change. What happened to the woman who couldn't abide irritating noisy toys, and who didn't want any plastic junk around? I seem to remember she never thought she'd eat anything that had been in another person's mouth. And she never, ever wanted to co-sleep with a child. She didn't like to be touched very much, because it made her skin sore. She was very clean about her person and thought she wouldn't let anybody ever, ever eat off her floor. She was definitely going to use cloth diapers all the time on her baby. She was going to make ALL the baby's food and was not going to humor picky eating. She hated the idea of a pacifier. She was going to give birth naturally, and to nurse the baby at her breast for two years. She knit, wrote fiction, made (and wore!) perfume, maintained her ties to the world. Her. Wow, where the hell did she go? Ah well, we don't need her. She didn't have a fucking clue what was going on in her world.
In her place, there's a woman in a milk-stained shirt and faded sweat pants, with unshaven legs and the distinct aroma of the unshowered. She has her ragged and torn hair up in a tight ponytail to avoid grabby hands: she looks like she trimmed her own bangs in desperation with nail scissors and the only product in them is baby drool and maybe a stray Cheerio. She's barefoot because she isn't going anywhere she doesn't have to go and because the bottoms of her shoes really aren't clean enough to bite, so they are exiled to the closet. Her toenail polish is chipped and has grown partly out; it's not that great a color anyway. Her knuckles are reddened and rough from washing up in hot water, and her nails are down to ragged stubs surrounded by hangnails, because they, too, are dry. Her unmoisturized arms are a solid mass of bruises, one with the very distinct impression of small toes.
She is grinning like a god-damned fool.