Fetal Development Lesson 4-9

Let’s view a baby’s growth from conception to birth. Take a peek inside the womb to see how a baby develops from month to month.

FERTILIZATION/CONCEPTION Fertilization happens when a sperm meets and penetrates an egg. It’s also called conception. At this moment, the genetic makeup is complete, including the baby’s sex, coloring, and facial features. Within about three days after conception, the fertilized egg, called a zygote, is dividing very fast into many cells. It passes through the fallopian tube into the uterus, where it attaches to the uterine wall. This is called implantation. The placenta, which will nourish the baby, also starts to form.

4 WEEKS The zygote becomes an embryo between 3- 8 weeks. The embryo is inside a thin, fluid-filled membrane called the amniotic sac, which surrounds and protects it. At this point the baby is developing the structures that will eventually form his/her face and neck. The heart and blood vessels continue to develop, and the lungs, stomach, and liver start to develop. A home pregnancy test would show positive.

8 WEEKS The embryo is now a little over half an inch in size. Eyelids and ears are forming, and you can see the tip of the nose. The arms and legs are well formed. The fingers and toes grow longer and more distinct. Notice the umbilical cord, a rope-like structure that connects the embryo and the mother’s placenta.

12 WEEKS Sometime between 9-14 weeks the embryo becomes a fetus. The fetus measures about 2 inches and starts to make its own movements. The doctor may hear the baby’s heartbeat with special instruments. The sex organs of the baby should start to become clear.

16 WEEKS The fetus now measures about just over 4 inches and weighs about 3.5 ounces. The baby’s eyes can blink, it develops eyebrows and eyelashes. The heart and blood vessels are fully formed. The baby’s fingers and toes have fingerprints.

20 WEEKS The baby weighs about 10 ounces and is a little more than 6 inches long. The baby can suck a thumb, yawn, stretch, and make faces. The mother may already feel the baby moving, which is called “quickening.”

TIME FOR AN ULTRASOUND An ultrasound is usually done at 20 weeks. During this ultrasound the doctor will make sure that the placenta is healthy and attached normally and that the baby is growing properly. You can see the baby’s heartbeat and movement of its body, arms, and legs on the ultrasound. You can usually find out if it’s a boy or a girl at 20 weeks.

24 WEEKS The baby weighs about 1.4 pounds now and responds to sounds, like music and conversations, by moving or increasing his/her pulse. The mother may feel jerking motions if the baby hiccups. With the inner ear fully developed, the baby may be able to sense being upside down in the womb. The baby probably also has a scheduled sleeping pattern.

28 WEEKS The baby weighs about 2 pounds 6 ounces, and changes position often at this point. If the mother delivered prematurely now, there is a good chance (90 %) that the baby would survive.

32 WEEKS The baby weighs about 4 pounds and is moving around often. The baby’s skin has fewer wrinkles as a layer of fat starts to form under the skin. Between now and delivery, the baby will gain up to half his/her birth weight. Sometime between weeks, the baby can use all 5 senses and has dreams. Most women go to the doctor every two weeks at this stage of pregnancy.

36 WEEKS Fetuses differ in size depending on many factors such as gender, the number of babies being carried, and the size of the parents. On average, a baby at this stage is about 18.5 inches and weighs close to 6 pounds. The brain has been developing rapidly. Lungs are nearly fully developed. The head is usually positioned down into the pelvis by now. A pregnancy is considered “at term” once it’s past 37 weeks.

BIRTH A mother’s due date marks the end of her 40th week. The delivery date is calculated using the first day of her last period. Based on this, pregnancy can last between 38 and 42 weeks with a full-term delivery happening around 40 weeks.

GENETICS: HEREDITY, passing traits from parent to offspring. Genes contain instructions that help determine a certain characteristic of an individual. DNA the genetic information that makes up chromosomes, tiny threadlike structures that carry heredity information from generation to generation Chromosomes are divided into smaller units called genes. Every cell contains 46 chromosomes, 23 from the mother and 23 from the father TRAIT a characteristic that an organism can pass to its offspring through its genes

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