Mothers who sleep with their baby in the same bed beyond six months are almost twice as likely to suffer depression, according to new research.
Women that co-bedded longer were more likely to worry over their baby's sleep and think they are being criticised - putting a strain on their relationships, researchers at Penn State University.
On average, these mothers reported being about 76 per cent more depressed than those who had moved their baby into a separate room.
Though the study authors did not take a side in the co-sleeping debate, they pointed out that sharing a bed may not only cause stress, but make it more difficult for parents to sleep well, making then more vulnerable to depression.
The 'baby' blues are a common occurrence for new mothers, as hormonal changes and pure exhaustion combine to work against their mental health.
But postpartum depression can be a much more serious and lasting condition.
While about 80 percent of women get the so-called blues, only 15 percent of births are accompanied by a mother's postpartum depression (PPD), which usually sets in between a week and a month after a baby's birth, and can last for months.
Stress can put mothers at greater risk of developing symptoms - like hopelessness, spontaneous crying, and even violent thoughts - so, efforts to minimize their worries and feelings of insecurity may help to prevent (PPD)
Dr Douglas Teti, lead author for the Penn State University study, said: "We definitely saw the persistent co-sleepers - the moms that were still co-sleeping after six months - were the ones who seemed to get the most criticism."
"They also reported greater levels of worry about their baby's sleep, which makes sense when you're getting criticized about something that people are saying you shouldn't be doing, that raises self-doubt. That's not good for anyone."
Co-sleeping with babies is a sensitive subject, with many parents feeling it is natural, or the only way to get their children to sleep is by lying in the same bed with them.
According to the Centers for Disease Control and Prevention (CDC), more than 60 percent of Americans report sharing their beds with their babies at some point.
Though the American Academy of Pediatricians stops short of saying that co-sleeping is safe, in 2016 the association revised its guidelines to acknowledge that, realistically, exhausted mothers are going to fall asleep with their babies in beds occasionally.
But the official advice is that the practices comes with an increased risk of infant death.
This can make mothers who choose to do it more likely to feel depressed or judged, said Dr Teti.
He said, regardless of current parenting trends, it is important to find a sleep arrangement that works for everyone in the family.
Prof Teti said: "In other parts of the world, co-sleeping is considered normal, while here in the US, it tends to be frowned upon."
"Co-sleeping, as long as its done safely, is fine as long as both parents are on board with it. If it's working for everyone, and everyone is okay with it, then co-sleeping is a perfectly acceptable option."
The researchers said most American families begin co-sleeping when their babies are first born, moving them to their own room by the time they are six months old.
Concerns about sudden infant death syndrome (SIDS) or the desire for babies to learn how to fall asleep on their own may be why many prefer them to sleep alone. An analysis of 103 mothers in their baby's first year of life saw a similar pattern.
Dr Teti said: 'We found about 73 percent of families co-slept at the one-month point. That dropped to about 50 percent by three months, and by six months, it was down to about 25 percent.
'Most babies that were in co-sleeping arrangements in the beginning were moved out into solitary sleep by six months.'
Dr Teti said the study published in Infant and Child Development was not about whether co-sleeping is good or bad, but the importance of finding an arrangement that works well without neglecting a partner or spouse.
He said: "If you are going to co-sleep, you have to make sure both people in the partnership have talked it through and both people are in sync with what they want to do."
"If not, that's when criticism and arguments can happen, and possibly spill over into the relationship with child. So you want to avoid that. You need to make sure you have time with your partner, as well."
Dr Teti also said that even when co-sleeping works well, it can still cause more loss of sleep for the parents than if the baby slept in its own room.
He added: 'If you co-sleep, it is going to disrupt your sleep, and probably Mom's sleep more than Dad's. So this is something to be careful with if you are not good with chronic sleep debt.
"Co-sleeping needs to work well for everyone, and that includes getting adequate sleep. To be the best parent you can be, you have to take care of yourself, and your child benefits as a result."
An estimated 3,500 babies die from sleep-related causes each year in the US, the CDC reported in January.
That number includes cases of suffocation, sudden infant death syndrome (SIDS) and deaths from unknown causes.
Last month, a Harvard University study reported that the rate of infants dying in their sleep has not changed in the last 25 years in the US, suggesting efforts to promote safer sleeping practice have gone unheeded.
The chance of sudden death goes up when bed-sharing if a parent smokes, has drunk alcohol or taken drugs, or is very tired, they say.
Babies are at greater risk of dying during co-sleeping if they were either premature or had a low birth weight.
The CDC recommends the safest place for a baby to sleep in the first six months is in a cot in the same room as the parent or carer.