Your Baby’s Development: The First Trimester

From the moment of conception to the time of delivery, your growing baby goes through several stages of development before he or she is ready to be born. Here’s what happens during the first trimester of pregnancy.

When does pregnancy begin?

Pregnancy begins when a sperm fertilizes a woman’s egg. Since you don’t always know when fertilization happened, doctors calculate pregnancy based on the first day of your last menstrual period (LMP). Your doctor will count forward 40 weeks from the LMP to estimate your due date (most births occur between weeks 38 and 42). If you have an ultrasound to look at your developing baby early in your pregnancy, your doctor may measure the baby to determine your due date instead.

What happens after the sperm fertilizes the egg?

After conception, your baby begins a period of dramatic change known as the embryonic stage. This stage runs from the 5th through the 10th week of the first trimester of pregnancy. During this stage, the baby is called an embryo.

What changes occur during the embryonic stage?

During this stage, all of your baby’s major organs and body parts begin to develop. The cells of the embryo (called embryonic stem cells) multiply and change into the hundreds of different types of cells needed to make a whole human body.

The placenta also develops during the embryonic stage. The placenta takes nutrients, oxygen and water from your blood and passes these along to your baby through the umbilical cord. It also removes the baby’s wastes. The placenta will filter out some—but not all— of the harmful substances that may be present in your body.

The amniotic sac develops during this stage, as well. This sac is filled with amniotic fluid, and forms inside your uterus to surround and protect your baby.

Some of the highlights of your baby’s development during the embryonic stage include:

  • Nervous system.This is one of the first things to develop during the embryonic stage. This means your baby’s brain, spinal cord and nerves form during this stage.
  • Heart.An S-shaped tube forms on the front of the embryo. This will become your baby’s heart. At first the heart does not beat, but soon it starts beating and pumping an early form of blood.
  • Face.Your baby’s facial features take shape during the embryonic stage. The eyes and ears form on the sides of the head and are linked to the brain. The eyes move forward on the face, and eyelids form to protect the developing eyes. Pieces of tissue grow and join together to create the forehead, nose, cheeks, lips and jaw. The nasal passages, mouth, tooth buds for the baby’s first teeth, and a tongue with taste buds also develop.
  • Arms and legs.At first, your baby’s arms and legs begin as little buds sprouting from the embryo’s sides. As they grow, the arms look like paddles and the legs like flippers. A ridge appears on the end of each one, eventually becoming fingers and toes.
  • Sexual organs.Cells that will become your baby’s eggs or sperm form. Your baby’s penis or vagina is visible at the end of the embryonic period, but it’s not yet possible to tell on an ultrasound if your baby is a girl or a boy.
  • Muscles and movement.Muscles develop and the embryo begins to move. At first it’s only twitching and reacting to touch. Once the nerves and muscles start working together, your baby can start moving on purpose.

By the end of the embryonic stage at week 10, the embryo will be about 1 inch long. That’s still too small for you to feel your baby’s movements. You’ll probably feel them starting in the middle of the second trimester.

What happens after the embryonic stage?

After the embryonic stage, the fetal stage begins and your baby is called a fetus. During the fetal stage, which runs from the 11th week until birth, your baby will grow longer and gain weight fast, and organs and body parts will continue to develop.

The last 2 weeks of the first trimester are the beginning of the fetal stage. In those weeks, fingernails and toenails form and the kidneys start working. By the end of the 12th week, which is the end of the first trimester, your baby has tripled in length to about 3 inches long.


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