Your Baby’s Development in the Second Trimester

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5 min read

Apr 16, 2025

What Does the Second Trimester Mean?

The second trimester of pregnancy spans from week 13 to week 26. It is often considered the most comfortable phase for many expectant mothers. The early pregnancy symptoms, such as nausea and fatigue usually subside while the baby continues to grow at an impressive rate.

This trimester is a critical period for fetal development as major organs and systems become fully functional, and significant growth milestones are achieved.

Development of Your Baby Month by Month

During the second trimester, the baby changes from a tiny, fragile form to a more defined and

active fetus. Each month brings developmental changes, preparing the baby for the final trimester and eventually to face the outside world.

Month 4 (Weeks 13-16)

By the start of the fourth month, the baby is about the size of an avocado, measuring around 4 to 5 inches long. During this stage:

The baby’s facial features become more distinct, with eyebrows, eyelashes, and hair beginning to develop.
The nervous system is rapidly developing which allows the baby to make more coordinated movements.
The baby’s bones start the deposition of calcium replacing the initial cartilage structure.
The reproductive system continues to develop. At this stage, the sex of the baby can be determined through an ultrasound.

At this stage, some mothers start to feel their baby move.

Month 5 (Weeks 17-20)

The fifth month is a remarkable period of growth. The baby is now about the size of a bell pepper, measuring 6 to 10 inches long and weighing close to a pound.

The baby’s skin is covered in a protective, waxy coating called vernix caseosa. It prevents damage from amniotic fluid.
A fine layer of hair called lanugo covers the baby’s body.
The baby’s hearing develops significantly, allowing them to recognize sounds, including the mother’s heartbeat and voice.
Movement becomes more pronounced, and parents may start feeling kicks and jabs.
The baby’s digestive system begins to function.

This month is also when most women undergo the mid-pregnancy ultrasound. It provides a detailed look at the baby’s development and it also allows parents to find out the baby’s gender if they wish.

Month 6 (Weeks 21-26)

By the end of the second trimester, the baby measures around 12 inches long and weighs about 1.5 to 2 pounds. This is an important period of development as the baby prepares for the third trimester.

The baby’s brain experiences rapid growth, forming billions of nerve connections.
Lungs begin producing surfactant. It is a substance which prevents the lungs from collapsing soon after birth.
The baby’s eyes start to open, and they may respond to light and darkness.
The baby continues swallowing and sucking.
Sleep cycles develop.

Conclusion

The second trimester is an exciting time in pregnancy as it shows rapid growth. By the end of this phase, the baby has transformed from a small, fragile being into an active and responsive fetus.

This trimester allows the parents to develop a deeper connection with their baby as the fetus has started to be more responsive to the external stimulus.

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