The baby’s heart begins to beat between 18-25 days. Brain waves begin around 40 days.
The placenta forms to provide the embryo with nourishment. The embryo’s heart begins beating with its own blood, sometimes a different type than either parent. The spine, nervous system, and internal organs begin to form. Small balls of cells that will grow into the baby’s muscles and bones appear. At 4 weeks, sensory cells begin to form eyes, ears and nose, and arms start to bud out.
All major organs have begun to form. The eyes have a retina and lens. Hair follicles, knees and elbows appear. A brain wave is detectable. The muscular system is working, and the baby is swimming and kicking. Fingers are noticible on the hands.
This is the last week for the baby to be called an embroyo. After this he or she is termed a fetus-Latin for “young one.” Fingerprints are forming, the baby begins to hear, and her responses to stimuli increase.
All organs are present. They will mature through the rest of the pregnancy. Cartilage begins to be replaced by bone.
The baby’s teeth and fingernails start to form. Baby boys begin producing testosterone. Genital organs are recognizable. The fetus can make a tight fist, and she can suck her thumb. She begins to make urine. You may be able to feel when she kicks or hiccups.
See the perfection of the baby’s feet and legs in these photos at 11 and 12 weeks. Notice how translucent the skin is at this point.
Week 13-16 (Month 4)1
From week 13-16 the fetus will grow from about 3 inches to 6 inches long. Her muscles are getting longer and her bones are hardening.
Her eyebrows and eyelashes develop. By now she can hear her mother’s voice and other sounds, and she can feel pain.
You can usually feel the baby’s movements as she grows bigger and stronger. By the end of week 16, her heart is pumping 25 quarts of blood per day
Week 17-20 (Month 5)1
By week 17, the baby can dream. By week 20 she is recognizing her mother’s voice. A fine hair called lanugo covers her body. Sex organs become more obvious. Notice the eyebrows, hair and fingernails on the five-month-old babies shown.
1 Pictures and data obtained from “Baby Development.” Let Her Live website. Retrieved July 14, 2010 fromwww.letherlive.org/development.php.
3D Ultrasound of a 28 Week Fetus
Picture obtained from NewScientist.com
Retrieved July 19, 2010, from http://www.newscientist.com/gallery/dn16745-visible-embryos/15.
“This week your baby weighs about 2 ½ pounds and measures around 15 inches long. During the last four weeks the weight of your baby has nearly doubled. The baby is moving around a lot these days, but soon this will slow down as the baby starts to run out of room inside your uterus.”2
2 “Fetal Development Week by Week-Week 28.” Retrieved July 19, 2010, fromhttp://www.amazingpregnancy.com/pregnancy-articles/213.html.
“36-Week 3D Ultrasound: The baby is filling out and has more fat on his face and other features.”
“During this time your baby continues his rapid weight gain, gaining about half a pound per week (mom will usually gain about a pound a week). By week 36, the average baby will weigh almost six pounds and measure close to 19 inches in length.”3
3 Doubilet, P. & Benson, C. (18 November, 2008). “Third Trimester Infant Development.” Just the Facts Baby. Retrieved July 20, 2010, from http://www.justthefactsbaby.com/pregnancy/article/third-trimester-infant-development/57.