What fetal developments occur at 35-39 weeks gestation?

From the Guidelines

At 35-39 weeks gestation, the fetus undergoes final preparations for birth, with significant developments in weight gain, lung maturation, brain development, and preparation for extrauterine life. During this period, the fetus continues to gain weight at a rate of about half a pound per week, reaching approximately 6-8 pounds by full term 1. The lungs complete their maturation with increased surfactant production, which is essential for proper breathing after birth. The brain continues rapid development, with neurons forming connections and the cerebral cortex adding layers. The digestive system matures as the fetus practices swallowing amniotic fluid and passing meconium, which will become the first bowel movement after birth. Some key points to consider in the management of pregnancies at this stage include the use of antenatal corticosteroids for individuals at risk for late preterm delivery 1, and the importance of monitoring fetal growth restriction, as small-for-gestational-age babies are at higher risk of neurodevelopmental delay, childhood and adult obesity, and metabolic disease 1. The fetus typically assumes a head-down position in preparation for delivery, with the head becoming engaged in the pelvis. Reflexes such as sucking, grasping, and blinking are well-established. The bones of the skull remain soft and unfused to allow for compression during vaginal delivery. Fat continues to accumulate under the skin, helping with temperature regulation after birth. These final developmental milestones ensure the baby is prepared for extrauterine life, with most organ systems functionally mature by 39 weeks. Key considerations for healthcare providers include the timing of delivery, with recommendations for delivery at 37 weeks of gestation in pregnancies with fetal growth restriction and an umbilical artery Doppler waveform with decreased diastolic flow but without absent/reversed end-diastolic velocity or with severe fetal growth restriction with estimated fetal weight less than the third percentile 1. Additionally, the use of Doppler assessment and cardiotocography testing are important tools in monitoring fetal well-being and guiding delivery decisions 1. Overall, the period of 35-39 weeks gestation is critical for fetal development and preparation for birth, and healthcare providers must carefully monitor fetal growth and well-being to ensure the best possible outcomes for mother and baby.

From the Research

Fetal Development at 35-39 Weeks Gestation

• At 35-39 weeks gestation, the fetus is considered late preterm, and its development is focused on preparing for life outside the womb 2.
• During this period, the fetus's lungs continue to mature, and the production of surfactant, a substance that helps the lungs expand and contract properly, increases 3, 4, 5.
• The fetus's brain and nervous system are also fully developed, and it can now regulate its own body temperature, which is essential for life outside the womb.
• The fetus's skin is thicker, and fat layers are formed, helping to regulate its body temperature after birth.
• The fetus can now open and close its eyes, and its eyelids are no longer fused shut.
• The fetus's digestive system is mature, and it can now practice breathing, swallowing, and digesting, preparing for life outside the womb.

Organ Development

• The fetus's lungs are mature, and the air sacs are fully formed, allowing for proper gas exchange after birth 4, 5.
• The fetus's heart is fully developed, and it can now pump blood efficiently, preparing for life outside the womb.
• The fetus's liver is mature, and it can now produce bile, which is essential for digestion.
• The fetus's kidneys are fully developed, and they can now produce urine, which is excreted into the amniotic fluid.

Sensory Development

• The fetus can now detect light and darkness, and its eyes are sensitive to light 2.
• The fetus can now hear sounds outside the womb, and its ears are fully developed.
• The fetus's sense of touch is fully developed, and it can now detect movement and pressure.
• The fetus's sense of taste and smell are also fully developed, and it can now detect different flavors and odors.