Braxton Hicks Contractions: Timing in Pregnancy
Braxton Hicks contractions can begin as early as the second trimester (around 18-24 weeks gestation), though they are most commonly perceived by pregnant women in the third trimester. 1, 2, 3
When Do Braxton Hicks Contractions Start?
Braxton Hicks contractions have been documented on ultrasound and tocography as early as 18-24 weeks of gestation, with research studies specifically examining these contractions during this gestational age range using real-time ultrasonography and Doppler evaluation 3
Most pregnant women begin to perceive these contractions during the third trimester, as they become more intense and frequent with advancing gestational age 1, 2
The contractions represent coordinated uterine activity that increases progressively toward delivery, distinguishing them from the uncoordinated Alvarez waves seen earlier in pregnancy 4
Clinical Characteristics That Distinguish Braxton Hicks from True Labor
Key distinguishing features include:
Irregular timing and pattern - unlike true labor contractions which become regular and progressively closer together 1
Subside with rest - Braxton Hicks contractions typically diminish when the pregnant woman stops activity and rests 1
Generally uncomfortable but not painful - they lack the progressive intensity and pain characteristic of true preterm or term labor 1
Do not cause cervical change - unlike true labor contractions which cause progressive cervical effacement and dilation 1
Important Clinical Pitfalls
Warning signs that require immediate medical evaluation:
Regular and painful uterine contractions that do not subside with rest may indicate preterm labor rather than benign Braxton Hicks contractions and require cessation of activity and immediate healthcare provider consultation 1
Women with high-risk conditions including active preterm labor, cervical insufficiency, or severe pre-eclampsia should avoid activities that stimulate uterine contractions 1, 5
Physiologic Impact on Fetal Well-Being
Braxton Hicks contractions can affect fetal heart rate parameters, with research showing that women who perceive these contractions demonstrate higher baseline fetal heart rate (135 bpm vs 128 bpm), lower long-term variability, and reduced number of accelerations on cardiotocography 2
Subplacental myometrium shows different contractile responses compared to non-placental myometrium during Braxton Hicks contractions, with arcuate arteries supplying placental portions maintaining stable resistance while non-placental areas show significantly increased resistance 3
Uteroplacental blood flow resistance increases considerably during contractions, with pulsatility index rising from 0.71 to 1.14, though this represents normal physiologic adaptation rather than pathology 6