Fetal Development Stages: Embryo to Fetus, in Weeks

Congratulations on becoming pregnant! We are sure you are curious about how your pregnancy will progress, and how your baby will develop week to week over the next few months. In this slideshow we will look inside the womb to see how a baby develops through the first, second, and third trimesters.

Step one of conception is when the sperm penetrates the egg to complete the genetic make-up of a human fetus. At this moment (conception), the sex and genetic make-up of the fetus begins. About three days later, the fertilized egg cell divides rapidly and then passes through the Fallopian tube into the uterus, where it attaches to the uterine wall. The attachment site provides nourishment to the rapidly developing fetus and becomes the placenta.

After 4 weeks, the basic structures of the embryo have begun to develop into separate areas that will form the head, chest, abdomen, and the organs contained within them. Small buds on the surface will become arms and legs. A home pregnancy test should be positive at this stage of development (most tests claim positive results one week after a missed period).

At 8 weeks, the fetus is about a half-inch long (1.1cm). Facial features such as developing ears, eyelids, and nose tip are present. The limb buds are now clearly arms and legs, while the fingers and toes are still developing.

At 12 weeks, the fetus has grown to about 2 inches (4.4cm) in length and may begin to move by itself. The fingers and toes are discernible, and the fetal heartbeat may be audible by Doppler ultrasound. The developing sex organs may be identified by ultrasound techniques.

At 16 weeks, the fetus is about 4 and one-half inches long and resembles an infant; the eyes blink, the heartbeat is easier to locate, facial features (nose, mouth, chin and ears) are distinct, and the fingers and toes are clearly developed; the skin on the fingers and toes even has distinct patterns (fingerprints!). Women should be able to feel the uterus at about 3 inches (6.6 cm) below the belly button; this is the beginning of the "baby bump" (abdominal swelling due to an expanding uterus) in some women.

At 20 weeks, the developing fetus is about 6 inches long (13.2 cm) and may weigh about 10 ounces. The baby may begin to make movements that the mother can feel at about 19 to 21 weeks; this baby movement is termed "quickening". The baby at this stage of development can move its facial muscles, yawn, and suck its thumb. The expanding uterus at 20 weeks is felt at the level of the belly button.

In the US, women that have prenatal care usually have an ultrasound done at 20 weeks to determine that the placenta is attached normally and that the baby is developing without any problems. The baby’s movements can be seen with Doppler imaging, and usually the sex of the baby can be determined at this time, so if you want to be surprised about the sex of your baby at delivery, let your doctor know before the Doppler ultrasound is started!

Shown here is a 2D ultrasound (inset) contrasted with a 4D ultrasound, both at 20 weeks.

At 24 weeks, the baby may weigh 1.4 pounds and can respond to sounds. Doppler studies show the sound response by measuring movement and heartbeat rates. Sometimes the baby will develop hiccups that the mother can feel! The baby's inner ear canals are developed at 24 weeks, so researchers speculate the fetus can sense its position in the uterus.

At 28 weeks, the baby normally weighs about 2 and one-half pounds and has developed to the point that if the baby is birthed prematurely for any reason, the chances are good that the infant will survive, but usually would require a hospital stay. Your doctor may discuss signs of premature labor and suggest you (and your partner) take classes on what to do at the time of delivery of your full-term baby.

At 32 weeks, many babies weigh about 4 pounds, and have movements that the mother can feel. Your doctor may ask you to make notes about the baby's movements and discuss breastfeeding and other options along with scheduling visits every two weeks until you deliver the baby. Some women begin to leak a yellowish fluid from their breasts around this time; this is normal. This fluid is termed colostrum, and its presence indicates the breasts are primed to start producing milk for the newborn baby.

At 36 weeks the baby is about ready to be delivered and has reached an average length of 18.5 inches from head to heel length and weighs about 6 pounds. However, a baby's weight and length are quite variable and are influenced by the baby's genetics, the baby's sex, and many other factors. During this time, the baby has begun to rotate itself into the delivery position of head first into the pelvis. At 37 weeks, the baby has completed development of all organ systems to a level that should allow it to survive and continue its growth outside the uterus without the close hospital monitoring that is usually done with premature babies; consequently, the pregnancy is considered "at term" at 37 weeks and beyond.

Delivery, due or birth date is calculated by estimating a 40 weeks delivery date, calculated after the first day of the mother's last period. This is an estimated date; the normal vaginal delivery birth can occur easily between 38 and about 42 weeks and is considered an early or late term pregnancy. However, most babies are delivered before 42 weeks. Depending on various circumstances and complications, the doctor may need to induce labor and delivery in some women, while others may require a surgical delivery (Caesarean section or C-section). For most people, especially first-time parents, birth of an infant is a life-changing event!