Mums can tell how well their child is developing from smell alone, according to new research that also found they can tell when they are about to start puberty.
The new study published in the journal Frontiers in Psychology involved having mothers smell the slept in T-shirts of their own and other children.
Researchers from Dresden University of Technology in Germany and others found a mother's nose is extremely sensitive to changes in her offspring's odours.
In the study of 164 German mothers, researchers found they could detect the developmental stage in their own child from scent alone.
• She tried to drown her disabled son: Auckland mother sentenced to home detention
• TOP STORY: Mother's 45 minutes of hell
• Martin Marinovich found guilty of his mother's murder
• Favona, Auckland, homicide: Innocent mother gunned down in a case of 'mistaken identity'
The pool of volunteers was tested with body odour samples of their own and four unfamiliar - but sex-matched - children who varied in age from baby to 18.
The study involved cotton T-shirts and onesies that children slept in for one night.
Lead author Dr Laura Schafer said it revealed that children's body odours are an important factor affecting the mother-child relationship.
"It hints toward its importance for affection and caregiving," she added.
Overall, about 64 per cent of mums were able to properly identify the development stage of the child whose T-shirt they sniffed.
They generally scored higher when identifying odours from children before puberty than from those who had gone through or were going through puberty.
The mums found that smells from younger children were nicer than those from older children who had already started puberty.
"This suggests infantile body odours can mediate affectionate love towards the child in the crucial periods of bonding," said Schafer.
Identifying a child as having gone through puberty "could be interpreted as a mechanism for detachment", according to the research team.
"This is when the child becomes more independent and separates itself from parental care," Schafer said.
The study follows previous experiments by the same team that used MRI scans to see how maternal brains react to baby odours.
These showed neural responses to baby odours were similar to other MRI brain studies that tested for facial cuteness.
Basically mums found baby smells cute.
The new study doesn't explicitly compare body odour to other forms of sensory stimulation but says it adds further evidence than "olfactory stimuli are an important factor in the mother-child bond".
The team is continuing to investigate the effects of body odour on the psychological relationship between mother and child.
They say their findings could have long-term clinical implications including ways to help mothers who struggle to bond with their infant after childbirth.
"It could lead to the development of methods such as neuro-feedback with olfactory stimuli or olfactory training to promote the rewarding component of body odours," said chafer.
A mother's emotional relationship with her baby begins during pregnancy.
Her feelings - described as bonding - typically grow and intensify after the baby's birth and become the foundation of the mother's relationship with her child.
Some studies have noted an association between postpartum depression and poor bonding, which is something Schafer and the team hope to help.
One found women with postpartum depression had prolonged difficulties with bonding compared to women who were not depressed.
But poor bonding is not universal among women with postpartum depression. new mothers who are not depressed may also experience bonding difficulties.
Schafer said her results open the door to an intervention that combines effective touch and olfactory stimulation.
The research suggests a child's body odour does convey developmental cues.
However, how the human nose is actually capable of sniffing out that information remains a mystery as hormonal levels do not appear to be a factor in how a mother determines the developmental stage of a child.
Schafer said the study is limited in that not all the factors that influence body odours, such as food or culture, can be accounted for in the current design.
In the future, she said they want to track individual changes to find further evidence for the results achieved in this smaller study.
They want to map indicators of the transition to puberty, and to find out whether this is reflected in the maternal perception of body odour.
Another similar piece of research by a Japanese team identified the scent of a baby as the key to helping women deal with the fraught first few months of motherhood.
Giving birth rewires the female brain so the odour of an infant fires up the part which keeps people rational, thoughtful and caring for others.
Another study by Canadian scientists found a baby's scent also raises levels of the pleasure messenger dopamine in the centre of the brain.
The smell sparks the same reward circuits as eating good food - or even having sex.
The research has been published in the journal Frontiers in Psychology.