Baby in the 2nd TrimesterThe Second Trimester of Pregnancy – Fetal Development


Your baby will grow in exciting ways during gestational weeks 13-27,  the second trimester of pregnancy.  He will experience tremendous fetal development now that the foundational body structures are in place.  You have more energy and the start of a baby belly forming.  There is so much happening on the outside, but have you thought about everything going on inside the womb right now?

What started as a cluster of cells has continued multiplying and grown into a fetus.  There are functioning organs, nerves, and muscles now. The heartbeat can now be heard easily with a Doppler at your appointments.

Remember that every pregnancy and baby is different, so your baby may develop a bit quicker or slower than this timeline.  Trust in your ability to grow and nurture this baby while he is inside of you.



Week 13 – 15

The placenta and umbilical cord are now providing oxygen and nutrients for your baby.  She is using breathing movements by utilizing the amniotic fluid to practice ‘breathing’ in and out.

Her heart is still beating fast; faster than it will towards the end of pregnancy and after she is born.

Tissue that will become bone is developing in the head and throughout the arms and legs.

Her ears and eyes are still developing and will continue to move forward on the head into their permanent places.

The skin is covered in a fine hair called lanugo.

Her neck is lengthening.

Hands and fingers are becoming functional, meaning she can suck her thumb.

Not only is your baby ‘practice-breathing’ amniotic fluid, but he is now ‘drinking’ it as well.  This is how the muscles learn to work once introduced to the real world, they practice with the amniotic fluid.

She can urinate.  Yes, your baby can now pee. The intestines have moved from the umbilical cord, where they were forming the last few weeks) to the abdomen.  Urine can form and be dispensed back into the amniotic fluid.

Your baby may be able to hear you.  His middle ear bones are hardening.

Red blood cells are forming in the spleen.

Your baby’s sex will become apparent during this time.  Ovarian follicles form on girls, and the prostate appears on boys.

Your baby’s scalp and hair pattern are forming.

His legs are longer than his arms, and he is now large enough for you to possibly start feeling him use them!

The fingernails and toenails are growing.

His skin is still opaque and very thin.

He is now about 3.5 inches long.



Weeks 16-17

An ultrasound would show you just how intense your baby’s movements are right now!  She is flipping and marching all over in there – most moms say they can feel their babies during this period.

Her eyes are now facing forward.

Fat is beginning to form under the skin.

Her neck is strengthening.

She is sucking, swallowing, and blinking throughout the days now.

The umbilical cord thickens and there is possible meconium backing up in the bowels.

Tissue continues hardening to bone.

You may feel little hiccups!

Fat continues accumulating, which will help to regulate baby’s body temperature after birth.

Baby is now just over 5 inches long.


Weeks 18-202nd Trimester pregnant lady

Vernix begins to form over the skin.

The placenta continues to grow.

The lungs and vocal cords develop alveoli.

The gender is more accurately predicted by ultrasound at this point.

Your baby may start to react to sounds or voices outside the womb.

A baby girl’s uterus and vagina are forming.

Her permanent teeth are forming behind the baby teeth that are already under the gums.

She begins sleeping on the same (or similar) schedule she will follow as a newborn.

She now measures just over 6 inches in length.


Week 20

The half-way point!

80-90% of babies reveal their gender through ultrasound this week.

The uterus is continually expanding to accommodate your baby.

Her growth begins to slow down around this week and level out to an even growth rate for the remainder of pregnancy.

Her nerve cells are developing to experience new senses.

She measures about 6.5 inches in length from crown to rump, but measurements at this point are taken from the crown of the head to heel of the foot, so she’ll average about 10 inches in length.


Weeks 21-23

Your baby’s white blood cells begin to form.

His skin is still opaque, and he is now swallowing easily.

His tongue is developing.

The fine hair (Lanugo) is now fully covering your baby. (This helps keeps the vernix on her.)

Her eyelids and eyebrows are complete.

A boy is forming sperm and testosterone, with his testes moving into place.

Your baby’s body proportions are almost normal now.

His eyes are fully formed, but they do not have color yet.

His skin is wrinkled and fingerprints are forming.

A girl’s ovaries now hold a lifetime supply of eggs.

Your baby weighs about a pound now.


Weeks 24-25

Your baby’s real hair is growing, unless she is meant to be a bald baby. Both outcomes are normal.

The taste buds of the tongue are continuing to develop, as are the lungs.

She is now officially viable, meaning she has a chance at surviving outside the womb with medical help now. She will have a 15% chance of survival. This percentage increases each day she is in the womb.

Rapid Eye Movement (REM) occurs during her sleep cycles.

Your baby’s position and movements put intense pressure on your bladder, causing frequent bathroom trips and making for unpleasant sleep positions.

Your baby’s spine is still forming and his nostrils are now opening.

He can wiggle his fingers and toes.

He is just over 13.5 inches long and close to a pound and a half in weight.


Diagram of lungs and alveoliWeeks 26-27

Your baby’s lungs are beginning to produce surfactant, the substance that allows the air sacs in the lungs to inflate.

Her finger and toenails are complete.

Her retinas are formed and she can open her eyes and blink.

There are detectable brain waves in response to hearing and sight.

Her nervous system is continuing to mature.

Your baby is about 14.5 inches long.