The sixth month of pregnancy typically spans from week 24 through the end of week 27, marking the final stage of the second trimester. This period is characterized by rapid physical growth and intense internal system maturation. This overview covers the average fetal size and key developmental milestones during this phase.
Defining the Average Fetal Size and Weight
During the sixth month, the fetus undergoes a significant increase in both length and mass. At the start of this period, around 24 weeks, the average length from crown to heel is approximately 12.7 inches (32.2 cm), and the weight is around 1.5 pounds (670 grams). This length is comparable to a large ear of corn.
By the end of the sixth month, at 27 weeks, the average fetus has grown to about 14.4 inches (36.6 cm) long and weighs roughly 2.3 pounds (1055 grams). The weight gain is substantial, nearly doubling over the four weeks, a pattern that continues into the third trimester. This size is often likened to a head of cauliflower.
These figures represent averages, and individual fetal growth can vary based on genetic factors and maternal health. Healthcare providers use these averages as a benchmark to ensure growth is progressing within a normal range. Measurements slightly above or below these numbers do not necessarily indicate an issue.
Key Functional Developments
The rapid physical growth during the sixth month is mirrored by significant progress in the development of the body’s major systems. The sensory system becomes more active, as the fetus can respond to sounds occurring in the mother’s surroundings. The hearing apparatus is well-developed, allowing the baby to recognize familiar voices and noises.
Internally, the lungs begin a crucial phase of maturation. Around 26 weeks, the air sacs start to produce surfactant. Surfactant is a substance that prevents the tiny air sacs, called alveoli, from collapsing when the baby exhales after birth. Though still immature, this production is a major step toward respiratory independence.
The central nervous system, particularly the brain, is experiencing rapid growth. Connections between nerve cells are becoming more complex, contributing to coordinated movements, sleep-wake cycles, and the ability to perceive stimuli. The baby also begins to accumulate brown fat, which is specialized adipose tissue that helps regulate body temperature after birth.
Methods for Monitoring Growth
Healthcare providers employ specific techniques to monitor fetal development and ensure the baby’s growth aligns with the expected trajectory. A common screening method is the measurement of fundal height, which is the distance in centimeters from the pubic bone to the top of the uterus. After 20 weeks, this measurement in centimeters should generally correspond to the number of weeks of gestation, plus or minus a few centimeters.
If the fundal height measurement is significantly different than expected, an ultrasound is often used to get more precise biometrical data. Ultrasound allows the provider to measure specific parts of the fetal anatomy.
Ultrasound Measurements
- Biparietal Diameter (BPD): The width of the head.
- Abdominal Circumference (AC).
- Femur Length (FL).
These individual measurements are then combined to calculate an Estimated Fetal Weight (EFW). The EFW is often plotted on customized growth charts to determine the baby’s growth percentile, comparing the baby’s size to other fetuses at the same gestational age. A baby whose estimated weight falls below the 10th percentile may be diagnosed with fetal growth restriction, which prompts closer monitoring. Identifying deviations in growth early allows for timely intervention if necessary.