Timeline of a Baby’s Prenatal 1st Trimester Development
Here’s a timeline of the preborn baby’s growth and development, according to EHD.org. Dating is measured from fertilization:
Week 1: The embryo attaches to the uterine wall, and the placenta begins to form.
Week 2: Early on in the first trimester, the baby’s brain is the first organ to appear, with its three main divisions, and the “beginnings of the heart can be seen.”
Week 3: The blood and blood vessels appear. (Right around this time is when the mother could have a positive pregnancy test.) The heart begins to beat between days 16 and 21.
Weeks 4 and 5: Eyes, lungs, and the cerebral hemispheres of the brain appear and begin growing quickly. Kidneys form.
Weeks 6 and 7: Brain waves begin, and hands, feet, and legs appear and begin to move. The heart now has four chambers, and the baby can rotate his head and have hiccups. Ovaries (for girls) and testes (for boys) form.
Weeks 8 and 9: The fetus stage begins. A girl now has eggs in her ovaries, and a baby at this stage can suck his thumb, move his tongue, sigh, stretch, move his head, and open his mouth. According to EHD, “right- and left-handedness emerges.”
Weeks 10-12: Still in the first trimester, the sex of the baby becomes visible, his nose and lips are “completely formed,” he can make facial expressions, and his intestines are “absorbi[ng] water and glucose.”
A human baby at 8 weeks: