Image by Camylla Battani

Learn what your body is doing week-by-week from fertilization to birth. 

Fetal Development

First Trimester

Conception—this is typically 2 weeks past the first day of your last period. The egg and sperm most often unite in the Fallopian tube to form a single cell called a zygote. In this tiny cell is all the information needed to determine the baby's features, including sex, and hair and eye color. 

DAY 2-5

This life is now called an embryo and it's cells are continuously dividing as it travels down the fallopian tube to the uterus. The lining of the uterus is preparing to receive the embryo. 

WEEK 1 (FROM FERTILIZATION)

The embryo implants on about day 6. Hormones are now triggered to nurture the pregnancy and prevent monthly periods. 

WEEK 2

A pregnancy test can now be taken to measure HCG, the pregnancy hormone, and tell the mother she is pregnant. The embryo is completely attached and draws nourishment from the mother. 

WEEK 3

The heart is the first organ to function and the first signs of brain development are evident. The foundation for every organ system is established and beginning to develop.

WEEK 4

Eyes are developing. Arm and leg buds are now visible. A beating heart can be seen on an ultrasound scan.

WEEK 5

The embryo measures 1/3 of an inch long and he or she is making its own blood. Depending on gender, the testicles or ovaries are beginning to form. 

WEEK 6

Elbows and fingers can be seen. Lungs are beginning to develop. Taste buds and tooth buds are forming, as well as eyelids. 

WEEK 7

Developing ears and nose are visible. There is pigment in the retina. Bones in the arms are beginning to calcify and harden. 

WEEK 8

The embryo is now known as a fetus and he or she can yawn and suck. Each minute the brain produces almost 250,000 new neurons! For the first time in development, the brain can make the muscles move on purpose. Upper and lower portions of arms and legs are clearly seen, as well as fingers and toes. A baby boy begins to produce testosterone. 

WEEK 9

The fetus is nearly 2 inches long and the kidneys are already producing urine. During the next several weeks, the baby's body will grow rapidly, increasing in weight 30x and tripling in length. 

WEEK 12

Now 3 1/2 inches long, the fetus is coordinated to find thumb and suck it. Fingernails and toenails are beginning to grow, and the baby can swallow and urinate.