The growing uterus is one of the most remarkable changes during pregnancy, and tracking its ascent in the abdomen is a common point of focus for expectant parents. The expansion of the uterus from a small, pelvic organ into the abdominal cavity signals the healthy progression of fetal growth and development. This upward movement causes the abdomen to noticeably swell, serving as a visible sign that the pregnancy is advancing. Since the belly button, or umbilicus, is a fixed and easily identifiable landmark, it serves as a simple reference point to gauge how far the uterus has expanded.
Tracking Fetal Growth Using Fundal Height
Healthcare providers monitor the rate of uterine expansion using a clinical measurement known as fundal height (FH). Fundal height is the distance, measured in centimeters, from the top of the pubic bone (symphysis pubis) to the highest point of the uterus, called the fundus. This simple, non-invasive measurement is a standard method used to screen for appropriate fetal growth during routine prenatal visits. After the 20th week of pregnancy, the fundal height measurement in centimeters often correlates closely with the number of weeks of gestation, providing a quick check on the baby’s development.
The measurement is typically taken with the pregnant person lying on their back, using a flexible tape measure stretched over the curve of the abdomen. Tracking this measurement over time helps confirm a steady pattern of growth, which is more informative than any single reading. While it is not a precise diagnostic tool like an ultrasound, fundal height offers a valuable, affordable screening method for growth irregularities.
The Typical Timeline for Reaching the Umbilicus
The uterus reaches the level of the umbilicus, or belly button, at a specific point in the pregnancy timeline. The fundus typically reaches this abdominal landmark around the 20th week of gestation. This milestone marks the approximate halfway point of a full-term pregnancy and represents a significant transition, as the uterus was previously confined within the bony structure of the pelvis.
As the uterus expands, it pushes upward and becomes an abdominal organ. At approximately 16 weeks, the fundus is usually located halfway between the pubic bone and the umbilicus. Reaching the belly button at 20 weeks is a reliable point of reference because it is the stage when the fundal height measurement begins to closely match the gestational age in weeks. This is also the point when the pregnancy often becomes outwardly visible.
Growth Progression After the Umbilicus
Once the fundus has reached the umbilicus at 20 weeks, its upward journey continues steadily throughout the second and into the third trimester. The uterus expands at a rate of approximately one centimeter per week, maintaining the rough correspondence between fundal height and gestational age. This continued growth means the fetus is moving further up into the abdominal cavity, displacing other internal organs.
By around 36 weeks of gestation, the fundus reaches its maximum height, typically extending to the base of the rib cage or the sternum’s xiphoid process. This high placement can cause physical sensations such as shortness of breath, as the expanded uterus crowds the diaphragm and lungs. Heartburn can also become more noticeable due to the upward pressure on the stomach. Near the end of the pregnancy, a process known as “lightening” occurs, where the baby’s head descends into the pelvis in preparation for birth. This descent causes the fundal height measurement to slightly decrease as the uterus moves lower than its 36-week peak.
Understanding Variations in Fundal Height
While the 20-week mark for reaching the umbilicus is a general guideline, various factors can cause an individual’s fundal height measurement to differ from the expected correlation. Maternal body type, particularly pre-pregnancy weight or obesity, can affect the accuracy of the measurement by making the fundus more difficult to palpate. The presence of uterine fibroids can also cause the measurement to be higher than anticipated for the gestational age.
The amount of fluid surrounding the baby significantly influences the measurement. Excessive amniotic fluid (polyhydramnios) can lead to a higher-than-expected fundal height, while too little amniotic fluid (oligohydramnios) may result in a smaller measurement. Fetal positioning, such as a breech presentation or a baby lying sideways, can also influence where the fundus is felt. Because of these common variations, slight differences in fundal height are usually considered normal, and healthcare providers focus on the consistent pattern of growth over time rather than a single measurement.