Fetal Development

First Trimester:

Week 2
Conception is the moment at which the sperm penetrates the ovum. Once fertilized it is called a zygote, until it reaches the uterus 3-4 days later.

Week 4
The embryo may float freely in the uterus for about 48 hours before implanting. Upon implantation, complex connections between the mother and embryo develop to form the placenta.

Week 6
The embryo is about 1/5 of an inch in length. A primitive heart is beating. Head, mouth, liver, and intestines begin to take shape.

Week 10
The embryo is now about 1 inch in length. Facial features, limbs, hands feet fingers and toes become apparent. The nervous system is responsive and many of the internal organs begin to function.

Second Trimester:

Weeks 13-16
The brain is fully developed and the fetus can suck, swallow, and make irregular breathing sounds. Fetus can feel pain. Fetal skin is almost transparent. Muscles tissue is lengthening and bones are becoming harder. Liver and organs produce appropriate fluids. Eyebrows and eyelashes appear and the fetus makes active movements including kicks and even somersaults.

Week 20
“Quickening” (when the mother can feel the fetus moving) usually occurs around this time. Finger and toenails appear. Lanugos, a fine hair now covers the entire body. The fetus can hear and recognize the mother’s voice. Sex organs are visible on ultrasound devices

Third Trimester:

Week 24
A protective waxy substance called Vernix covers the skin. By birth, most of the vernix will be gone but any that is left is quickly absorbed. Fetus has a hand and startle reflex. Footprints and fingerprints are forming. Fetus practices breathing by inhaling amniotic fluid into its developing lungs.

Weeks 25-28
Rapid brain development occurs during this period and the nervous system is able to control some bodily functions. The fetus’ eyelids now open and close. At 25 weeks there is a 60% chance of survival if born. The fetus is considered legally viable at 28 weeks and there is a 90% chance of survival if born at this point.

Weeks 29-32
There is a rapid increase in the amount of body fat the fetus has. Rhythmic breathing occurs, but the lungs are not yet mature. The fetus sleeps 90-95% of the day. At this point there the survival rate is above 95% if the baby is born.

Weeks 38-40
The fetus is considered full-term. Lanugos is gone except on upper arms and shoulders. Hair on the baby’s head is now coarser and thicker. The lungs are mature. The average weight of the baby at this point is seven and a half pounds. At birth the placenta detaches from the uterus and the umbilical cord will be cut as the baby takes his first breaths of air. Breathing will trigger changes in the heart and bypass arteries forcing all blood to now travel through the lungs