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Does pregnancy increase or decrease pain sensitivity?

Intuitively, it seems like it should. After all, rising levels of hormones like estrogen and progesterone during pregnancy are known to have analgesic effects in animal models. As someone who has given birth, I would say there should be some endogenous mechanism to suppress pain during pregnancy, considering the job at hand!  But has this been proven to be true?

Do pregnant women truly become less sensitive to pain, and if so, when and how?


In this blog, we’ll take a closer look at research that utilized Quantitative Sensory Testing (QST) to investigate how pain thresholds, tolerance, and brain responses change throughout pregnancy, labor, and postpartum. The findings may surprise you.

expecting mother with excited big brother
Expecting mother with excited big brother

Does pain change throughout the pregnancy?


Heat pain thresholds during pregnancy


Draisci and colleagues (Draisci, 2012) tested pregnant women (20 in each group) in two different stages of pregnancy: 32-33 gestational weeks (GW), 39-40 GW, at the early stages of active labor (regular contractions and dilation <3) and 24 hours after delivery and compared them to non-pregnant nulliparous control women (n=20). QST was performed both at the forearm as well as the abdomen at T10 dermatome level using Pathway ATS 30x30 mm thermode. They found that heat pain thresholds progressively increased throughout the pregnancy (32 and 40 GW), and were significantly different between the non-pregnant women both at the forearm as well as at the abdomen.


Conditioned Pain Modulation and Temporal Summation during pregnancy


In a later longitudinal study including 33 women, Carvalho et al. tested conditioned pain modulation (CPM), mechanical temporal summation (TS) of pain, and the temperature to induce supra-threshold pain at numerical rating scale 6/10 as the test-stimulus for CPM. This study included women in different phases of their pregnancy (8-12 GW, 18-22 GW, 36 GW and 6-12 weeks post-partum (Carvalho B. G., 2016). Mechanical TS was induced using 180 monofilaments, while CPM was induced using a test-stimulus of contact heat at pain level 6/10 with Medoc’s TSA-II, and a conditioning stimulus of a hot bath at 46.5°C.


Even though mTS decreased throughout the pregnancy from an average score of 3.81 in the first trimester to 1.56 in the last trimester, this difference was not statistically significant. Equally so, Pain 6/10 temperature increased during the pregnancy from an average of 46.88 in the first trimester to 47.5 in the third trimester, but these changes were not statistically significant. The CPM score did not change significantly between stages of the pregnancy either. 


Pain tolerance during pregnancy


Pain tolerance may change during pregnancy Carvalho et al. also found (Carvalho B. A., 2006). When comparing 20 pregnant women (GW 39) to 19 non-pregnant women for heat pain tolerance using the TSA-II with 16x16 mm thermode, they found a small (1°C) but statistically significant (p<0.05) difference between the pregnant women pre-labor to the nulliparous women. However, this difference did not “hold water” in the cold pressor bath for pain tolerance as measured by the number of seconds the subject could hold their hand in the water, nor was it significant for cold pain threshold with the thermode. It need be noted that the nulliparous group was much younger than the pregnant group and significantly lighter in weight.


Leźnicka et al. tested pressure pain thresholds and pressure pain tolerance with Medoc’s AlgoMed in pregnant women in GW 13-28 and compared it to non-pregnant volunteers. Pressure pain tolerance on the dominant and non-dominant hand was significantly lower in pregnant women than it was non-pregnant women. (Leźnicka, 2022)


Do pain thresholds change during labor?


Draisci et al. who explored heat pain thresholds through the pregnancy and early childbirth found that these thresholds were at their highest during early childbirth as compared to 32 and 40 GW pregnancy, and to non-pregnant controls both at the forearm as well as on the abdomen (p<0.001). (Draisci, 2012)


Does pain sensation normalize after childbirth?


Draisci et al. found that 24 hours after labor, pain thresholds on average decreased from the early active labor 42.3 to 40.4, however, they did not yet reach the same levels as controls 40.1 at the abdomen, with similar differences at the forearm. (Draisci, 2012) 


In the longitudinal study by Carvalho et al., the temperature to elicit pain at the 6/10 level decreased from an average of 47.5 during the third trimester to 46.9 in the 6-12 weeks post-partum, however this difference was not statistically significant. (Carvalho B. G., 2016)  


In testing pain tolerance for heat, women 1-2 days post-partum still had significantly higher heat pain tolerance than nulliparous women. In cold pain tolerance, there was no difference between women who had recently given birth and those who were not pregnant. (Carvalho B. A., 2006) 


Do contact heat evoked potentials (CHEPs) differ between pregnant and non-pregnant women?


Draisci et al. compared women in different stages (32 GW, 40 GW) of their pregnancy to nulliparous non-pregnant women for N2-P2 amplitude, N2 and P2 latencies at two body sites, the forearm and the abdomen, using Pathway CHEPS. (Draisci, 2012) 

Pain ratings of the CHEP stimulation were collected and found to be significantly different at both testing sites (right forearm and abdomen) between week 32 and week 40, with women at week 40 being less sensitive than in week 32.


The N2-P2 amplitude both at the abdomen and on the forearm was not different between GW 32 and non-pregnant women. In GW 40, the N2-P2 amplitude decreased significantly as compared to GW 32 on both body sites (p<0.001) and was also significantly different from the non-pregnant volunteers for the abdomen (p<0.03).


As for the N2-P2 latencies in pregnant women, they did not differ from those in non-pregnant women at either site, nor did they change significantly between gestational weeks 32 and 40.


Are we more or less sensitive to pain during pregnancy?


To conclude, even though there are mixed results between the different studies, some studies had methodological issues like significant age differences between the pregnant and non-pregnant group, as well as major weight and height differences. Most studies point to increasing analgesia as the pregnancy progresses, coming to a peak near the end and during labor, with some normalization post-partum.



References

 

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