During the second and third trimesters of pregnancy, you may notice
episodes when your belly tightens and becomes firm to the touch, then relaxes.
These are episodes of tightening (contraction) of the uterine muscles called
Braxton Hicks contractions. These normal contractions may be hardly noticeable
or may be strong enough to make you stop what you are doing.
Considered "warm-up" exercises for the uterus, Braxton Hicks
contractions can begin as early as the 20th week of pregnancy, although most
often they start between the 28th and 30th week.
Braxton Hicks contractions are usually infrequent in mid-pregnancy.
However, they can be more frequent during the ninth month, sometimes occurring
as often as every 10 to 20 minutes.footnote 1
Braxton Hicks contractions:
Usually disappear during exercise or activity
(unlike true labor pains, which continue or increase if you move
Are more noticeable during rest.
It may be hard to tell the difference between Braxton Hicks
contractions and true labor. If there is any doubt, consult your health
Cunningham FG, et al. (2010). Maternal physiology. In Williams Obstetrics, 23rd ed., pp. 107-135. New York: McGraw-Hill.