Pregnancy
FAQ156, April 2018



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How Your Fetus Grows During Pregnancy

How does pregnancy begin?

Fertilization, the union of an egg and a sperm into a single cell, is the first step in a complex series of events that leads to pregnancy. Fertilization takes place in the fallopian tube. Over the next few days, the single cell divides into multiple cells. At the same time, the small cluster of dividing cells moves through the fallopian tube to the lining of the uterus. There it implants and starts to grow. From implantation until the end of the eighth week of pregnancy, it is called an embryo. From the ninth week of pregnancy until birth, it is called a fetus.

What is the placenta?

The placenta is formed from some of these rapidly dividing cells. The placenta functions as a life-support system during pregnancy. Oxygen, nutrients, and hormones from the mother are transferred across the placenta to reach the fetus, and waste products from the fetus are transferred to the mother for removal.

How will my uterus change during pregnancy?

During pregnancy, the lining of your uterus thickens and its blood vessels enlarge to provide nourishment to the fetus. As pregnancy progresses, your uterus expands to make room for the growing fetus. By the time your baby is born, your uterus will have expanded to many times its normal size.

How long does pregnancy last?

A normal pregnancy lasts about 40 weeks from the first day of your last menstrual period (LMP). Pregnancy is assumed to start 2 weeks after the first day of the LMP. Therefore, an extra 2 weeks is counted at the beginning of your pregnancy when you are not actually pregnant. Pregnancy “officially” lasts 10 months (40 weeks)—not 9 months—because of these extra weeks.

How is the length of my pregnancy measured?

Pregnancy can be divided into weeks and sometimes days. A pregnancy that is “36 and 3/7 weeks” means “36 weeks and 3 days of pregnancy.” The 40 weeks of pregnancy often are grouped into three trimesters. Each trimester lasts about 12–13 weeks (or about 3 months):

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

What is the estimated due date?

The day your baby is due is called the estimated due date (EDD). Only about 1 in 20 women give birth on their due dates. Still, the EDD is useful for a number of reasons. It determines your fetus's gestational age throughout pregnancy so that the fetus's growth can be tracked. It also provides a timeline for certain tests that you will have throughout your pregnancy.

How is my estimated due date calculated?

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

What happens during weeks 1–8 of pregnancy?

  • Placenta begins to form.
  • The brain and spinal cord begin to form.
  • The tissues that will form the heart begin to beat. The heartbeat can be detected with ultrasound at about 6 weeks of pregnancy.
  • Buds for limbs appear with paddle-like hands and feet.
  • The eyes, ears, and nose begin to develop. Eyelids form, but remain closed.
  • The genitals begin to develop.
  • By the end of the eighth week, all major organs and body systems have begun to develop.

What happens during weeks 9–12 of pregnancy?

  • 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.
  • The arms are longer than the legs.

What happens during weeks 13–16 of pregnancy?

  • Arms and legs can flex.
  • External sex organs are formed.
  • The outer ear begins to develop.
  • The fetus can swallow and hear.
  • The neck is formed.
  • Kidneys are functioning and begin to produce urine.

What happens during weeks 17–20 of pregnancy?

  • The sucking reflex develops. If the hand floats to the mouth, the fetus may suck his or her thumb.
  • The skin is wrinkled, and the body is covered with a waxy coating (vernix) and fine hair (lanugo).
  • The fetus is more active. You may be able to feel him or her move.
  • The fetus sleeps and wakes regularly.
  • Nails grow to the tips of the fingers.
  • The gallbladder begins producing bile, which is needed to digest nutrients.
  • In female fetuses, the eggs have formed in the ovaries. In male fetuses, the testes have begun to descend.
  • It may be possible to tell the sex of the fetus on an ultrasound exam.

What happens during weeks 21–24 of pregnancy?

  • The fetus may hiccup.
  • The brain is rapidly developing.
  • Tear ducts are developing.
  • Finger and toe prints can be seen.
  • The lungs are fully formed but not yet ready to function outside of the uterus.

What happens during weeks 25–28 of pregnancy?

  • The eyes can open and close and sense changes in light.
  • The fetus kicks and stretches.
  • The fetus can make grasping motions and responds to sound.
  • Lung cells begin to make a substance that will enable breathing.

What happens during weeks 29–32 of pregnancy?

  • With its major development finished, the fetus gains weight very quickly.
  • Bones harden, but the skull remains soft and flexible for delivery.
  • The different regions of the brain continue to form.
  • Hair on the head starts to grow.
  • Lanugo begins to disappear.

What happens during weeks 33–36 of pregnancy?

  • The fetus usually turns into a head-down position for birth.
  • The brain continues to develop.
  • The skin is less wrinkled.
  • The lungs are maturing and getting ready to function outside of the uterus.
  • Sleeping patterns develop.

What happens during weeks 37–40 of pregnancy?

  • The fetus drops lower into the pelvis.
  • More fat accumulates, especially around the elbows, knees, and shoulders.
  • The fetus gains about half a pound per week during this last month of pregnancy.

Glossary

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 stage of prenatal development that starts at fertilization (joining of an egg and sperm) and lasts up to 8 weeks.

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 stage of prenatal development that starts 8 weeks after fertilization and lasts until the end of pregnancy.

Gestational Age: The age of a pregnancy, usually calculated from the number of weeks that have elapsed from the first day of the last normal menstrual period and often using findings from an ultrasound examination performed in the first or second trimester of pregnancy.

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.

If you have further questions, contact your obstetrician–gynecologist.

FAQ156: This information was designed as an educational aid to patients and sets forth current information and opinions related to women’s health. It is not intended as a statement of the standard of care, nor does it comprise all proper treatments or methods of care. It is not a substitute for a treating clinician’s independent professional judgment. Please check for updates at www.acog.org to ensure accuracy.

Copyright April 2018 by the American College of Obstetricians and Gynecologists

American College of Obstetricians and Gynecologists
409 12th Street SW, Washington, DC  20024-2188 | Mailing Address: PO Box 70620, Washington, DC 20024-9998