Lightening in Late Pregnancy: Fundal Height Changes
In the last month of pregnancy, lightening typically occurs when the height of the fundus (A) drops.
Understanding Lightening
Lightening refers to the descent of the fetal head into the maternal pelvis during late pregnancy, which causes the uterine fundus to drop in height. 1
Normal Fundal Height Progression
- The fundus reaches the level of the umbilicus at approximately 20 weeks gestation 1, 2
- By 36 weeks, the fundus typically reaches the level of the xiphisternum 1, 2
- In the last month of pregnancy (after 36 weeks), there may be a diminution of fundal height from 36 cm down to approximately 32 cm as the fetal head engages into the pelvis 1
The Mechanism of Lightening
- This descent occurs when the fetal presenting part (usually the head) descends into the maternal pelvis in preparation for labor 1
- The engagement of the fetal head causes the fundus—the top portion of the uterus—to drop lower in the maternal abdomen 1
- This physiological change is most notable in primigravidas (first-time mothers) and typically occurs in the last 2-4 weeks before delivery 1
Clinical Significance
The drop in fundal height during lightening is a normal physiological process and should not be confused with pathological causes of decreased fundal height. 1
Important Distinctions
- The cervix does not "drop" in height—it undergoes effacement (thinning) and dilation during labor 1
- The umbilicus is a fixed anatomical landmark on the maternal abdomen and does not change position 1, 2
- Only the fundus (the uppermost portion of the uterus) demonstrates measurable descent during lightening 1, 3