image of a fetus at 28 weeks of development

Frequently Asked Questions about Prenatal Development

by Brittany Spalding |

Picture of the bottom of a mom's feet, and 5 sets of baby's feet at 24, 28, 32, 36 and 40 weeks of developmentCongratulations! You’re expecting! In addition to breath-taking excitement, you may be overwhelmed with questions about what to expect during your pregnancy. This is especially true for first-time parents. Below, we have provided all the answers to the most frequently asked questions about prenatal development.

How does pregnancy begin?

Fertilization is the union of an egg and a sperm into a single cell in the fallopian tube. This is the first step in a complex series of events that leads to pregnancy. Throughout the next few days, the single cell divides into multiple cells and moves through the fallopian tube to the lining of the uterus where it implants and starts to grow.

As the fertilized egg grows, a water-tight sac forms around it, gradually filling with fluid. This is called the amniotic sac, and it helps cushion the growing embryo. From implantation until the end of the eighth week of pregnancy, your baby is called an embryo. From the ninth week of pregnancy until birth, it is called a fetus.

What is the placenta?

The placenta functions as a life-support system for your baby during pregnancy, and is formed from some of these rapidly dividing cells. The placenta is a round, flat organ that transfers nutrients, oxygen, and hormones from the mother to the baby, and transfers wastes from the baby.

How will my uterus change during pregnancy?

Your uterus will change drastically during pregnancy. The lining of your uterus will thicken, and its blood vessels will enlarge to provide nourishment to the fetus. As your pregnancy progresses, your uterus will expand to make room for your growing baby. By the time your baby is born, your uterus will have expanded to many times its normal size.

How long does pregnancy last?

The average healthy pregnancy lasts around 40 weeks from the first day of your last menstrual period (LMP). It is assumed to start 2 weeks after the first day of your LMP. This means that an extra 2 weeks is counted at the beginning of your pregnancy when you are not actually pregnant. Pregnancy technically lasts 10 months (40 weeks)—not 9 months—because of these extra weeks.

How is the length of my pregnancy measured?

The 40 weeks of pregnancy are commonly grouped into three trimesters. Each trimester lasts about 12–13 weeks, or approximately 3 months.

  • First trimester: 0 weeks – 13 weeks 6 days (Months 1–3)
  • Second trimester: 14 weeks – 27 weeks 6 days (Months 4 –7)
  • Third trimester: 28 weeks – 40 weeks 6 days (Months 7–9)

What is the estimated due date (EDD)?

The day your baby is due is called the estimated due date (EDD). Only about 5% of women give birth on their actual due dates. Still, the EDD is useful for a number of reasons because it determines your baby’s gestational age throughout pregnancy so your baby’s growth can be tracked. Additionally, it provides a timeline for certain tests that you will need to have throughout your pregnancy.

How is my EDD calculated?

Your EDD is calculated from the first day of your LMP. When the date of your LMP is uncertain, an ultrasound exam can be done during the first trimester to estimate the due date. If you’ve had in vitro fertilization, your EDD is set by the age of the embryo and the date that the embryo is transferred to the uterus.

What happens during weeks 1–4 of pregnancy?

Image of the features of a fetus at 4 weeks amazng

Baby at 4 weeks

Congratulations! You’re pregnant! The dividing fertilized egg moves down the fallopian tube toward the uterus. Around 5 days after fertilization, the cluster of dividing cells enters the uterus. Approximately 8–9 days after fertilization, the cluster of cells (now called a blastocyst) attaches to the lining of the uterus.

A primitive face will take form with large dark circles for eyes. The mouth, lower jaw, and throat are developing. Blood cells are taking shape, and circulation will begin. The tiny “heart” tube will beat 65 times a minute by the end of the fourth week. By the end of the first month, your baby is about 1/4 inch long – smaller than a grain of rice!

What happens during weeks 5–8 of pregnancy?

image of the fetus at 8 weeks of pregnancy

Baby at 8 Weeks

Your baby’s facial features continue to develop. Each ear begins as a little fold of skin at the side of the head. Tiny buds that eventually grow into arms and legs are forming. Fingers, toes, and eyes are also forming. Eyelids form, but remain closed. The neural tube (brain, spinal cord, and other neural tissue of the central nervous system) is well-formed. Bone starts to replace cartilage. The head is large in proportion to the rest of the baby’s body.

The genitals also begin to develop. By the end of the eighth week, all major organs and body systems have begun to develop. By the end of the second month, your baby is about 1 inch long and weighs about 1/30 of an ounce. At about 6 weeks, your baby’s heartbeat can usually be detected. After the 8th week, your baby is called a fetus instead of an embryo.

What happens during weeks 9–12 of pregnancy?

image of a fetus at 12 weeks

Baby at 12 Weeks

There are big changes taking place this month. Buds for future teeth appear. Fingers and toes start to form. Soft nails begin to form. Bones and muscles begin to grow. The intestines begin to form. The backbone is soft and can flex. The skin is thin and transparent. The hands are more developed than the feet, and the arms are longer than the legs.

What happens during weeks 13–16 of pregnancy?

image of a fetus at 16 weeks

Baby at 16 Weeks

Your baby’s neck, arms, hands, fingers, feet, and toes are fully formed and can flex. Your baby can open and close its fists and mouth. Fingernails, toenails, eyebrows, and eyelashes are beginning to develop, and the external ears are formed. By the end of this month, your baby is fully formed.

All the organs and extremities are present and will continue to mature in order to become functional. The circulatory and urinary systems are working, and the liver produces bile. Your baby can swallow and hear. External sex organs are formed, and the placenta is fully formed. In male fetuses, the testicles begin to descend from the abdomen. Genitals become either male or female at week 14.

By the end of this month, your baby is about 4 inches long and weighs about 1 ounce. Since your baby’s most critical development has taken place, your chance of miscarriage drops considerably after three months.

What happens during weeks 17–20 of pregnancy?

image of a fetus at 20 weeks

Baby at 20 Weeks

Your baby’s heartbeat may now be audible through an instrument called a doppler. The fingers and toes are well-defined. Eyelids, eyebrows, eyelashes, nails, and hair are formed; and teeth and bones become denser. The skin is wrinkled, and the body is covered with a waxy coating (vernix) and fine hair (lanugo). Lanugo protects your baby and is usually shed at the end of the baby’s first week of life.

The baby’s skin is covered with a whitish coating called vernix caseosa. This “cheesy” substance is thought to protect baby’s skin from the long exposure to the amniotic fluid. This coating is shed just before birth.

The sucking reflex develops during this time. If the hand floats to the mouth, your baby may suck his or her thumb. Your baby can even yawn, stretch, and make faces. The nervous system is starting to function. Your baby is more active, and you may be able to feel him or her move. The fetus sleeps and wakes regularly.

During this time, the gallbladder begins producing bile, which is needed to digest nutrients. The reproductive organs and genitalia are now fully developed, and your doctor can see on ultrasound if you are having a boy or a girl. In female fetuses, the eggs have formed in the ovaries. By the end of this month, your baby is about 6 inches long and weighs about 4 ounces.

What happens during weeks 21–24 of pregnancy?

image of a fetus at 24 weeks of development

Baby at 24 Weeks

This month, you will begin to feel your baby move more frequently, since he or she is developing muscles and exercising them. Real hair begins to grow, and the eyes begin to open. The brain is rapidly developing during this time. Your baby’s unique finger and toe prints can be seen.

The lungs are fully formed, but not yet functioning. By the end of this month, your baby is about 10 inches long and weighs from 1/2 to 1 pound. If born prematurely, your baby may survive after the 23rd week with intensive care.

What happens during weeks 25–28 of pregnancy?

image of a fetus at 28 weeks of development

Baby at 28 Weeks

During this time, the eyes can open and close and sense changes in light. Your baby can also make grasping motions and respond to sound. Your little one is more active this month – kicking and stretching more frequently than before. Lanugo begins to disappear, and the lung cells start producing surfactant. By the end of this month, your baby is about 12 inches long and weighs about 2 pounds.

What happens during weeks 29–32 of pregnancy?

Image of a fetus at 32 weeks of development

Baby at 32 Weeks

With the major development finished, your baby gains weight very quickly now. The bones harden, but the skull remains soft and flexible for delivery. The different regions of the brain are forming, and the taste buds develop. Your baby can now taste sweet and sour, and may even begin to hiccup. Your baby’s hearing is fully developed.

He or she changes positions frequently and responds to stimuli – including sound, pain, and light. The amniotic fluid begins to diminish. Your little one is about 14 inches long and weighs from 2 to 4 pounds. Your baby will continue to mature and develop reserves of body fat. If born prematurely during this time, your baby would likely survive.

What happens during weeks 33–36 of pregnancy?

image of a fetus at 36 weeks of development

Baby at 36 Weeks

Your baby will continue to mature and develop reserves of body fat, and you may notice that he or she is kicking more. Most internal systems are well developed, as the brain continues to develop. The lungs are maturing and getting ready to function outside the uterus, but they may still be immature.

During this time, the skin becomes less wrinkled, and sleeping patterns develop. The fetus usually stays in a head-down position in preparation for birth. Your baby is about 18 inches long and weighs as much as 5 pounds.

What happens during weeks 37–40 of pregnancy?

images of 9 babies, at the end of each moth of pregnancy

Fetal Development – Watching Baby Grow!

Your baby continues to grow and mature, and the lungs are nearly fully developed. Your baby’s reflexes are coordinated so he or she can blink, close the eyes, turn the head, grasp firmly, and respond to sounds, light, and touch. More fat accumulates, especially around the elbows, knees, and shoulders. Your baby is definitely ready to enter the world!

You may notice that your baby moves less due to tight space. Your baby’s position changes to prepare itself for labor and delivery. The baby drops down in your pelvis. Usually, the baby’s head is down toward the birth canal. The fetus gains about 1/2 pound per week during this last month of pregnancy. Your baby is about 18 to 20 inches long and weighs about 7 pounds.

Adverse Factors Affecting Fetal Development

  • Poor nutrition
  • Use of alcohol
  • Smoking
  • Use of certain prescription or over-the-counter drugs
  • Use of recreational drugs such as cocaine, sedatives, and narcotics
  • X-rays and other kinds of radiation
  • Ingested toxins, such as lead
  • Illnesses such as AIDS, German measles, syphilis, cholera, smallpox, mumps, or severe flu


Cell: The smallest unit of a structure in the body; the building blocks for all parts of the body.

Egg: The female reproductive cell produced in and released from the ovaries; also called the ovum.

Embryo: The developing organism from the time it implants in the uterus up to 8 completed weeks of pregnancy.

Fallopian Tube: One of a pair of tubes through which an egg travels from the ovary to the uterus.

Fertilization: Joining of the egg and sperm.

Fetus: The developing organism in the uterus from the ninth week of pregnancy until the end of pregnancy.

Gestational Age: The age of a pregnancy calculated from the number of weeks that have elapsed from the first day of the last normal menstrual period.

Hormones: Substances made in the body by cells or organs that control the function of cells or organs. An example is estrogen, which controls the function of female reproductive organs.

In Vitro Fertilization: A procedure in which an egg is removed from a woman’s ovary, fertilized in a laboratory with the man’s sperm, and then transferred to the woman’s uterus to achieve a pregnancy.

Oxygen: A gas that is necessary to sustain life.

Placenta: Tissue that provides nourishment to and takes away waste from the fetus.

Sperm: A cell produced in the male testes that can fertilize a female egg.

Surfactant: A substance produced by cells in the respiratory system that contributes to the elasticity of the lungs and keeps them from collapsing.

Trimesters: The three 3-month periods into which pregnancy is divided.

Ultrasound Exam: A test in which sound waves are used to examine internal structures. During pregnancy, it can be used to examine the fetus.

Uterus: A muscular organ located in the female pelvis that contains and nourishes the developing fetus during pregnancy.

Brittany Spalding
Brittany Spalding is a wife, homeschool mom, writer, editor, and food allergy awareness advocate. She enjoys spending time with her family, gardening, restoring antique furniture, St. Louis Cardinals baseball, and fishing on the beautiful Kentucky Lake.

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