Is sense of smell heightened during pregnancy?

So, is it true that sense of smell increases during pregnancy? We churn over the data with Professor Paul Breslin from the Monell Chemical Senses Centre in Philadelphia...

Paul - It's quite common for pregnant women to report that odours are stronger for them, that they're more sensitive to odours, that they're somewhat less tolerant of ambient room odours and that odours tend to be more unpleasant.

Laboratory studies have shown that pregnant women rate above-threshold odorants - those that are weak to moderately strong - as more intense than do non-pregnant women. There's little evidence, however, that pregnant women are more sensitive to odours in the absolute detection sense. That is, they're not able to detect the presence of odours at lower concentrations.

Hannah - So, it appears that the widely-reported claim that sense of smell heightens during pregnancy is not due to increased smell sensitivity, but actually, due to above-threshold smells appearing stronger and making them appear more disgusting. But why does this happen? Over to Tim Jacob, Professor of the Psychophysiology of Smell at Cardiff University.

Tim - This particularly happens in the first trimester of pregnancy and it's thought that there is some evolutionary advantage here in that it's necessary for the mother to be very careful what she ingests, not to ingest toxins and other poisons, both for her own health and for that of her foetus.

Hannah - So, what could be controlling this sense of smell during pregnancy? Back to Paul.

Paul - We have found that female hormones can greatly increase sensitivity to odours in women who are attending to them compared to females who are too young to cycle or compared to post-menopausal women. So, it appears that female hormones can influence the function of the olfactory system.

Hannah - Thanks, Paul and also do Adam who got in touch with the question. And finally, thanks to Tim who also points out that there are cases of expectant mothers with a decreased sense of smell. So, it's not a hard and fast rule for all...