- Any health problems for anyone concerned in the first couple months (which is when it's most likely, for the mother and baby in particular, obviously) will be really, really bad, as far as making those weeks much worse than they'd otherwise be.
- If you get very unlucky (sometimes) or suck at sleep training (way, way more often) get ready for years of bad sleep, yourself. If you're good at this the dreaded "sleep zombie" stage of parenting is only ~2-3 months long, though, so no big deal in the scheme of things, and even better if you've got a spouse who isn't the kind of person to "punish" their partner any time they have to be awake by making them also be awake (lots of people truly are like this, and I think it's nuts, if you do some good trading-off with one another, then the first couple months are actually really easy and you'll only be sleep-deprived like half the time, at worst)
- If your kid is sick, at all, then sleep in those early weeks will be exceptionally terrible, as kid illnesses tend to manifest in frequent waking & crying that can't always be assuaged with milk or rocking or whatever. Even a mild illness can make those early nights damn dark, and you may have trouble figuring out what's wrong (may take shopping for a doctor to find one who won't just rudely brush you off as a newbie parent with a "colicky" baby and actually listen to you and try to figure it out)
- They can be incredibly expensive, for all kinds of reasons, including but not limited to healthcare expenses (in the US), damage (ours have done over a thousand dollars of damage to stuff around our house in some years, more if we counted the rate of wear on e.g. flooring or the cost to pay for repairs on all kinds of things that we were able to do ourselves very cheaply), education (paid directly or, more commonly, in much higher housing expenses than you'd otherwise have). Oh and as they get older, food. (clothes and toys are negligible expenses compared to the rest, unless you don't want them to be)
Basically if no-one gets sick and you take sleep training seriously, they're just expensive and everything else is barely a problem... until they get old enough to start going places on their own, and then you get a whole new set of worries :-)
- If you get very unlucky (sometimes) or suck at sleep training (way, way more often) get ready for years of bad sleep, yourself. If you're good at this the dreaded "sleep zombie" stage of parenting is only ~2-3 months long, though, so no big deal in the scheme of things, and even better if you've got a spouse who isn't the kind of person to "punish" their partner any time they have to be awake by making them also be awake (lots of people truly are like this, and I think it's nuts, if you do some good trading-off with one another, then the first couple months are actually really easy and you'll only be sleep-deprived like half the time, at worst)
- If your kid is sick, at all, then sleep in those early weeks will be exceptionally terrible, as kid illnesses tend to manifest in frequent waking & crying that can't always be assuaged with milk or rocking or whatever. Even a mild illness can make those early nights damn dark, and you may have trouble figuring out what's wrong (may take shopping for a doctor to find one who won't just rudely brush you off as a newbie parent with a "colicky" baby and actually listen to you and try to figure it out)
- They can be incredibly expensive, for all kinds of reasons, including but not limited to healthcare expenses (in the US), damage (ours have done over a thousand dollars of damage to stuff around our house in some years, more if we counted the rate of wear on e.g. flooring or the cost to pay for repairs on all kinds of things that we were able to do ourselves very cheaply), education (paid directly or, more commonly, in much higher housing expenses than you'd otherwise have). Oh and as they get older, food. (clothes and toys are negligible expenses compared to the rest, unless you don't want them to be)
Basically if no-one gets sick and you take sleep training seriously, they're just expensive and everything else is barely a problem... until they get old enough to start going places on their own, and then you get a whole new set of worries :-)