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I've heard anecdotes from pregnant women that they have a heightened sense of smell, and How Target Figured Out A Teen Girl Was Pregnant Before Her Father Did cites a NYT article mentioning similar claims about preferring unscented products:

[Pole] ran test after test, analyzing the data, and before long some useful patterns emerged. Lotions, for example. Lots of people buy lotion, but one of Pole’s colleagues noticed that women on the baby registry were buying larger quantities of unscented lotion around the beginning of their second trimester. Another analyst noted that sometime in the first 20 weeks, pregnant women loaded up on supplements like calcium, magnesium and zinc. Many shoppers purchase soap and cotton balls, but when someone suddenly starts buying lots of scent-free soap and extra-big bags of cotton balls, in addition to hand sanitizers and washcloths, it signals they could be getting close to their delivery date. [emphasis added]

While I can see the evolutionary advantages in pregnant women being more sensitive to bad smells, I assume that our sense of smell would be limited by the number of receptors, which can't be increased.

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    My hypothesis (read: My guess) is that it may be due to some pre-pregnancy paranoia by many mothers to avoid chemicals, and them -- probably unconsciously -- associating strong scent with harmful chemicals. It's a bit like avoiding cigarettes and alcohols during pregnancy, but on a broader scale.
    – Lie Ryan
    Mar 16, 2012 at 11:11
  • anecdotal - my wife had a massively heightened sense of smell while pregnant with all 3 of our kids. Nothing to do with harmful chemicals - it affected everything she did and where she went, having to avoid any strong smells when shopping, working etc. She almost had to avoid the kitchen entirely when I was cooking.
    – Rory Alsop
    Mar 16, 2012 at 11:18
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    Even though I don't think the question really cites examples of the claim, Rory's anecdotes, plus those of friends I have heard (e.g. suddenly unable to walk down the laundry aisle at the supermarket) suggest it is notable.
    – Oddthinking ♦
    Mar 16, 2012 at 11:48
  • I would posit that it is not a heightened sense of smell rather a reduced tolerance of the sensations that strong smells cause.
    – Chad
    Mar 16, 2012 at 19:40

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Do pregnant women think they have a heightened sense of smell? Yes.

The results show that abnormal smell and/or taste perception was reported by 76% of the pregnant women, typically believed to be caused by their pregnancy. Increased smell sensitivity was found to be very common at the early stage of pregnancy (67% of all pregnant respondents) and occasionally accompanied by qualitative smell distortions (17%) and phantom smells (14%). The smell abnormalities were less common at the late pregnancy stage and almost absent post partum. Abnormal taste sensitivity was fairly commonly reported (26%), often described as increased bitter sensitivity and decreased salt sensitivity. These results, suggesting that abnormal smell and/or taste perception is experienced by a large majority of pregnant women, imply that further research is needed to understand to what extent these chemosensory changes may underlie food aversions and craving with implications for food intake during pregnancy.

Now, the question remains: Are they really more sensitive, or is it more that their opinion of the smells that change (i.e. things smell better, worse or just subjectively stronger?)

Well, the science is a little unclear:

These scientists surveyed ridiculously large numbers of women (13,610 pregnant and 277,228 nonpregnant).

In comparison to nonpregnant women, pregnant women rated their own sense of smell lower, more often rated the test odors less pleasant smelling, more often classified the test odors as inedible, were less likely to report odor-evoked memories, and used perfume and cologne less frequently. Differences in odor detection and intensity rating did not favor either group.

So, it seems that the smells aren't stronger, just subjectively less pleasant.

These scientists measured smell of over 100 women in early pregnancy, and got a similar result. Sense of smell wasn't stronger, but some smells were subjectively less pleasant. However, they also measured their sense of taste (gustatory) and found it had weakened.

Pregnant women had significantly lower overall gustatory sensitivity scores. There were no differences in olfactory sensitivity. However, pregnant women rated the odors ‘rum’, ‘cigarette’ and ‘coffee’ as more aversive than did non-pregnant women.

Conclusion: Our data do not support the hypothesis of a generalized increase in chemosensitivity in early pregnancy. In terms of adaptive changes of the olfactory system may act as a sentinel to potentially harmful chemicals. In contrast, the gustatory system appears to retreat to allow a greater intake of electrolytes and a more widely sourced diet.

To be fair, at least one set of scientists didn't get the same result, testing a smaller sample:

M. Laska, B. Koch, B. Heid and R. Hudson Failure to Demonstrate Systematic Changes in Olfactory Perception in the Course of Pregnancy: a Longitudinal Study Chem. Senses (1996) 21 (5): 567-571. doi: 10.1093/chemse/21.5.567

Olfactory function was assessed in 20 women during each trimester of pregnancy and post partum, and compared with that of 20 non-pregnant women tested in parallel. In contrast to earlier reports, no consistent differences in olfactory sensitivity or odor evaluation were found between the two groups.

Meanwhile, another scientist wasn't impressed with the research, to date in 2007:

Although considerable anecdotal evidence suggests that pregnancy affects olfactory sensitivity, scientific evidence is limited and inconclusive. Whereas hedonic ratings are affected by pregnancy, odor identification is not.

Dr Cameron tested 100 women in total (pregnant, postpartum, or had never been pregnant) against a standard smell identification test.

Mean UPSIT scores did not differ significantly across groups indicating no difference in odor identification. Trends in planned comparisons suggested that in the first trimester, odors were rated as more intense and less pleasant. In the first trimester, women scratched the odor strips significantly fewer times. Consistent with previous reports, 90% of pregnant women reported that specific odors smelled less pleasant and 60% reported that some odors smelled more pleasant. Although nearly two-thirds of pregnant women rated their olfactory sensitivity to be enhanced during pregnancy and overall pregnant women's self-rated olfactory sensitivity was higher than controls', self-ratings were not correlated with UPSIT scores nor odor intensity ratings.

These results suggest that these and previous findings may reflect the fact that the effect of pregnancy on olfaction is small and inconsistent.

So her results were largely consistent that pregnancy doesn't make you a bloodhound, even if it makes you feel like one.

Is it even biologically feasible that pregnancy would make your sense of smell different?

Yes, a paper in Science showed that hormones in pregnant rats stimulated the production of new olfactory interneurons.

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