How a Baby Grows During Pregnancy

Pregnancy begins when a male’s sperm enters a female’s egg (fertilization). Fertilization usually happens in one of the tubes (fallopian tubes) that connect the ovaries to the womb (uterus).

The fertilized egg moves down the fallopian tube to the uterus. Once it reaches the uterus, it implants into the lining of the uterus and begins to grow.

For the first 10 weeks, the fertilized egg is called an embryo. After 10 weeks, it is called a fetus. As the fetus continues to grow, it receives oxygen and nutrients through tissue (placenta) that grows to support the developing baby.

The placenta is the life support system for the baby. It provides oxygen and nutrition and removes waste.

Learning as much as you can about your pregnancy and how your baby is developing can help you enjoy the experience. It can also make you aware of when there might be a problem and when to ask questions.

How long does a typical pregnancy last?

A pregnancy usually lasts 280 days, or about 40 weeks. Pregnancy is divided into three periods of growth, also called trimesters:

• First trimester: 0–12 weeks.
• Second trimester: 13–27 weeks.
• Third trimester: 28–40 weeks.

The day when your baby is ready to be born (full term) is your estimated date of delivery.

How does my baby develop month by month?

First month

• The fertilized egg attaches to the inside of the uterus.
• Some cells will form the placenta. Others will form the fetus.
• The arms, legs, brain, spinal cord, lungs, and heart begin to develop.
• At the end of the first month, the heart begins to beat.

Second month

• The bones, inner ear, eyelids, hands, and feet form.
• The genitals develop.
• By the end of 8 weeks, all major organs are developing.

Third month

• All of the internal organs are forming.
• Teeth develop below the gums.
• Bones and muscles begin to grow. The spine can flex.
• The skin is transparent.
• Fingernails and toenails begin to form.
• Arms and legs continue to grow longer, and hands and feet develop.
• The fetus is about 3 inches (7.6 cm) long.

Fourth month

• The placenta is completely formed.
• The external sex organs, neck, outer ear, eyebrows, eyelids, and fingernails are formed.
• The fetus can hear, swallow, and move its arms and legs.
• The kidneys begin to produce urine.
• The skin is covered with a white, waxy coating (vernix) and very fine hair (lanugo).

Fifth month

• The fetus moves around more and can be felt for the first time (quickening).
• The fetus starts to sleep and wake up and may begin to suck its finger.
• The nails grow to the end of the fingers.
• The organ in the digestive system that makes bile (gallbladder) functions and helps to digest nutrients.
• If your baby is a girl, eggs are present in her ovaries. If your baby is a boy, testicles start to move down into his scrotum.

Sixth month

• The lungs are formed.
• The eyes open. The brain continues to develop.
• Your baby has fingerprints and toe prints. Your baby’s hair grows thicker.
• At the end of the second trimester, the fetus is about 9 inches (22.9 cm) long.

Seventh month

• The fetus kicks and stretches.
• The eyes are developed enough to sense changes in light.
• The hands can make a grasping motion.
• The fetus responds to sound.

Eighth month

• All organs and body systems are fully developed and functioning.
• Bones harden, and taste buds develop. The fetus may hiccup.
• Certain areas of the brain are still developing. The skull remains soft.

Ninth month

• The fetus gains about ½ lb (0.23 kg) each week.
• The lungs are fully developed.
• Patterns of sleep develop.
• The fetus’s head typically moves into a head-down position (vertex) in the uterus to prepare for birth.
• The fetus weighs 6–9 lb (2.72–4.08 kg) and is 19–20 inches (48.26–50.8 cm) long.

What can I do to have a healthy pregnancy and help my baby develop?

General instructions

• Take prenatal vitamins as directed by your health care provider. These include vitamins such as folic acid, iron, calcium, and vitamin D. They are important for healthy development.
• Take medicines only as directed by your health care provider. Read labels and ask a pharmacist or your health care provider whether over-the-counter medicines, supplements, and prescription drugs are safe to take during pregnancy.
• Keep all follow-up visits as directed by your health care provider. This is important. Follow-up visits include prenatal care and screening tests.

How do I know if my baby is developing well?

At each prenatal visit, your health care provider will do several different tests to check on your health and keep track of your baby’s development. These include:

• Fundal height and position.
  • Your health care provider will measure your growing belly from your pubic bone to the top of the uterus using a tape measure.
  • Your health care provider will also feel your belly to determine your baby’s position.
• Heartbeat.
  • An ultrasound in the first trimester can confirm pregnancy and show a heartbeat, depending on how far along you are.
  • Your health care provider will check your baby’s heart rate at every prenatal visit.
• Second trimester ultrasound.
  • This ultrasound checks your baby’s development. It also may show your baby’s gender.

What should I do if I have concerns about my baby’s development?

Always talk with your health care provider about any concerns that you may have about your pregnancy and your baby.

Summary

• A pregnancy usually lasts 280 days, or about 40 weeks. Pregnancy is divided into three periods of growth, also called trimesters.
• Your health care provider will monitor your baby’s growth and development throughout your pregnancy.
• Follow your health care provider’s recommendations about taking prenatal vitamins and medicines during your pregnancy.
• Talk with your health care provider if you have any concerns about your pregnancy or your developing baby.