When Daniel was about 2 weeks old, I officially gave up the idea of making him sleep on his back. He just wouldn't do it. He was tired, and I was tired. I felt like an evil selfish mom for letting him belly-sleep against all of the SIDS recommendations. When I mentioned it to my pediatrician and asked his opinion (he's generally super laid back) he said "well, if your baby died of SIDS could you live with yourself for letting him belly sleep?" I was dumbfounded. What a thing to say!
Still, I was so tired, as was he, that I continued to let him sleep on his belly. I'm not typically a worrying-type mother, but I would go in and check on him frequently, convinced I was setting him up for death. It wasn't until he was about 6 months old (about when the SIDS risk drastically reduces itself to nearly nothing) that I decided to do some research around belly-sleeping.
I do not wish to downplay the risk or tragedy of SIDS. I just want to point out some factors mommies often do not consider.
The official recommendations about no longer belly-sleeping came about because of the "Back to Sleep" campaign in the early 90s (previous recommendations were to make them sleep on their belly to avoid suffocation from spit-up). The scientific community noticed a huge correlation between the time the campaign ran and a reduction in SIDS deaths- an over 50% reduction by some accounts. That, and that alone, is the reason for the recommendation. I'm not doubting the correlation exists- I do believe less belly-sleeping will mean less SIDS. Just like other common-sense guidelines that were also a part of the Back To Sleep campaign (no soft bedding, and importantly- no smoking near baby) can also be tied to the reduction in infant deaths. But a correlation does not establish cause-and-effect. It's also important to note that while the number of SIDS diagnoses went down drastically, the overall number of infant deaths hardly changed- just the number of SIDS diagnoses (by the way, the only way to really diagnose SIDS is to rule out every other possible cause of death.) It's also important to note that during this period- where the reduction in SIDS deaths is tied to just back-sleeping- there was a huge cultural push for breastfeeding (shown to reduce your SIDS risk by 2 or 3 times) and stopping second-hand smoke, and could also be tied to the drop in SIDS deaths.
To this day, there is no official stance on what causes SIDS. Most lay people think it's a kind of suffocation- though officially a SIDS diagnosis rules out death by suffocation- that's a different diagnosis. Some think it's a genetic defect. Recent studies favor the idea that it's a form of sleep apnea. That makes the most sense to me, though I wouldn't be surprised if it's a combination of genetics and sleep apnea or maybe something else entirely.
But why, if it's apnea (or even genetics), would belly vs. back matter? Because SIDS affects babies when they're in their deepest form of sleep. And most babies sleep much more deeply on their stomaches. ALL of the SIDS guidelines (back-sleeping, keeping the room cooler, having a fan blowing on the baby- they've even noticed in studies a decrease in SIDS when caffeine was administered to the babies) have one thing in common: they keep the baby from sleeping as deeply.
Now, some babies are deep sleepers no matter what you throw at them. This, I believe, is why some babies still die of SIDS even while sleeping on their backs. Unfortunately, that means there isn't much their moms can do to reduce the risk of SIDS. You can follow every SIDS guideline and still lose your baby. The odds are low to begin with and lower if you follow all the guidelines- but there is no way to eliminate the risk altogether. Sorry if that's a depressing way to view it.
This study shows that babies who slept on their bellies had far more "efficient" sleep but also more instances of apnea (the non-fatal kind)- which goes hand-in-hand with the SIDS-is-linked-to-apnea-during-deep-sleep theory. Babies that were studied who later actually died of SIDS were much less likely to have arousals during their sleep than babies who were never affected by SIDS. In other words, belly-sleeping babies slept more deeply, and longer, than their back-sleeping counterparts. But the deep-sleepers were also at more risk for SIDS.
The problem is, not letting a baby sleep deeply carries its own set of risks. I'm borrowing this straight from the wikipedia article on the Back to Sleep campaign (I added the emphasis):
"...According to the study, at 6 months of age, the infants who were placed to sleep in the prone position had statistically significant higher social skills scores, gross motor scores, and total development scores than those infants who were put to sleep in the supine [back] position. [A different study] reported that "Typically developing infants who were sleep-positioned in supine had delayed motor development by age 6 months, and this was significantly associated with limited exposure to awake prone positioning."" (to be fair, it goes on to note that how much time the babies had on their tummies while awake was not factored into the study and might have an effect).
In other words, babies need that deep sleep for their development. Without deep sleep, they are likely to hit their milestones later. On top of that, back-sleeping leads to an increase in problems like cradle cap (not a huge deal), asphyxiation from reflux (a very big deal), and since the Back to Sleep campaign, there has been a huge increase in the amount of babies with plagiocephaly or cranial asymmetry problems (funky-shaped heads from laying on their soft skulls)- not to mention the temporary and less worrisome cradle bald spots.
In other words, there is a set of risks, whether you sleep your baby on the belly or their back. I won't judge a mother either way. The risk of SIDS for a normal, healthy baby is very small compared to the risks that come from back-sleeping, but obviously death is infinitely more tragic than an asymmetrical head or hitting milestones a few months later. I do think that parents of babies that are more prone to SIDS (respiratory problems, prematurity, diagnosed apnea, snoring...) should consider follow all possible SIDS guidelines to the letter. But if a mommy lets her baby sleep on his or her belly, it's possible that instead of being a negligent parent, they are choosing a different set of (scientifically based) benefits and risks- longer sleep periods, deeper sleep, and more likely to meet milestones on time.
If it's worth the risk to you, then go for it. I do. I personally feel that letting my baby sleep deeply is more important than the small risk of SIDS. But, I also understand that there's validity to valuing protection from SIDS above deep sleep. I really don't think you'd be "right" or "wrong" either way- or rather, I think both ways can be right, depending on what risks you're most comfortable with. No mother should ever feel judged for making an informed decision.