By 12 weeks in utero, a baby has already grown so much! The fetus is 2.5 to 3 inches long — about the size of a plum. At this stage of growth, they have a detectable heartbeat and brainwaves, as well as distinguishable features, genitals, and fingernails. This is just the beginning! Here’s a weekly breakdown of fetal development in the second trimester.
*Please note that physicians mark “pregnancy” as beginning from the first day of the woman’s last menstrual period. The woman is not actually pregnant during these first two weeks. Conception — actual pregnancy when the egg is fertilized — generally occurs sometime within the 2 weeks following the first day of the woman’s last menstrual period. Medical developmental terminology is noted in this article for easy reference to medical literature. However, the actual gestational age of the fetus from conception is then used.
Week 14 (12 weeks following conception): The fetus’ external genitals are fully developed and the sex of the baby is quite apparent. Skin begins to thicken, hair grows along the body, and mobility improves as the fetus begins moving its hands and head.
Week 15 (13 weeks following conception): Bones are becoming more visible via ultrasound and some organs are shifting to their permanent locations. The fetus’ lungs are beginning to develop, as it practices breathing using amniotic fluid. The fetus may now begin to smile or suck its thumb.
Week 16 (14 weeks following conception): The fetus can slowly move its eyes and may react to light. As the fetus’ ears move into their permanent location, they are developed enough to hear you talk.
Week 17 (15 weeks following conception): The fetus begins putting on fat and its skin takes on a thick, white coating of vernix — a substance intended to protect the fetus’ delicate skin from the amniotic fluid. The vernix arrives with perfect timing, as the fetus is now moving and rolling more frequently in the amniotic sac. The fetus’ heart is pumping about 100 pints of blood every day.
Week 18 (16 weeks following conception): The fetus’ digestive system is functional. The fetus also develops a sleep/wake cycle.
Week 19 (17 weeks following conception): You may begin feeling your baby kick in the womb! The fetus now has unique fingerprints.
Week 20 (18 weeks following conception): The part of the brain that receives signals from the five senses (parietal lobe) is beginning to develop.
By the end of the fifth month, the fetus weighs about a pound and is about 9 to 10 inches long.
Week 21 (19 weeks following conception): The fetus has bone marrow, which will later support the production of blood cells.
Week 22 (20 weeks following conception): The fetus’ eyebrows and hair are now visible via ultrasound. The fetus continues putting on fat, including brown fat, which supports heat production. Additionally, the fetus’ ears are continuing to develop, enabling them to hear your bodily sounds (heartbeat, breathing, etc).
Week 23 (21 weeks following conception): You may notice more jerky movements from your baby as they fight persistent hiccups.
Week 24 (22 weeks following conception): Your baby’s lungs are fully developed but not yet functional. The fetus’ skin has taken on a pink or red tone as blood-filled capillaries become more visible.
Week 25 (23 weeks following conception): As we near the end of the second trimester, the fetus’ nervous system is developing and it begins “filling out” as it puts on more body fat.
Week 26 (24 weeks following conception): As we approach the third trimester, the fetus makes melanin — a substance responsible for bringing pigment to the skin and eyes.
Learn more about the various stages of fetal development (conception, first, second, and third trimester) and don’t forget to follow us on social media (Facebook, Instagram, and TikTok).
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