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Stages of Pregnancy

Now that you know the gestational age of your pregnancy and have read the statement on genetics, you can, if you like, review a brief description of embryonic or fetal development for gestational age. However, you do not have to read the description of embryonic or fetal development for gestational age.

If you decide to terminate your pregnancy, you must sign a statement verifying that you were given the opportunity to review this information on embryonic and fetal development at least 24 hours in advance of pregnancy termination. This statement is available at the clinic and also on the DHEC website at www.scdhec.gov/Health/WRTK.

We’ve listed each two-week period under the appropriate trimester (as defined under South Carolina’s abortion law). Each two-week period is identified two ways:

  • By gestational age (the length of time that has passed since the first day of your last normal menstrual period)
  • By conceptional age, which is the time that has passed since actual conception. Conceptional age will always be about two weeks younger than gestational age. Under South Carolina’s abortion laws, trimesters are defined by conceptional age.

We've listed each two-week period under the appropriate trimester (as defined under South Carolina's abortion law). Each two-week period is identified two ways:

  • By gestational age (the length of time that has passed since the first day of your last normal menstrual period)
  • By conceptional age, which is the time that has passed since actual conception. Conceptional age will always be about two weeks younger than gestational age.
  • Under South Carolina's abortion laws, trimesters are defined by conceptional age.

A trimester is approximately 13 weeks, or one third of a typical full-term pregnancy.

First Trimester

0-2 Weeks from Conception (3-4 weeks after the last menstrual period)
The egg is released from the ovary.  It is fertilized in the fallopian tube by the sperm. The fertilized egg starts to divide and forms a ball of cells. The ball of cells digs into the lining of the uterus.

  • The ball of cells begins to form layers and fluid-filled spaces.
  • The earliest part of the afterbirth begins to form.
  • At this point in its growth, the ball of cells is called an embryo.
  • The embryo grows to a length of about 1/100th of an inch (0.2 mm).

3-4 weeks from conception (5-6 weeks after the last menstrual period)
The embryo changes from a flat disc to a curved, C-shaped form. Organs begin forming. At this point, the menstrual period is missed.

  • A tube forms along the embryo's length. This will grow into the brain and spinal cord.
  • The heart starts as a tube, which begins to beat as it grows.
  • Simple structures form on the sides of the head. They will become eyes and ears as time goes on.
  • Limb buds, which look like bumps, start to form. Later they will become arms and legs.
  • The embryo grows to a length of ¼ inch (6 mm).

5-6 weeks from conception (7-8 weeks after the last menstrual period)
About half of the embryo's length is the head, due to the rapid growth of the brain. The heart starts to form the normal four chambers. A heartbeat can be seen on ultrasound.

  • The eyes and ears move toward their normal places on the head.
  • Kidneys begin to form.
  • "Rays" appear in the limbs, which will later form fingers and toes.
  • The umbilical cord joins the embryo and the placenta (or afterbirth).
  • The embryo is about ½ inch long (14 mm).
  • The neural tube that becomes the brain and spinal cord closes

7-8 weeks from conception (9-10 weeks after the last menstrual period)
The embryo changes shape as the face forms. It begins to straighten out from its C-shape. The small tail bud begins to go away. All of the essential organs, including the basic parts of the brain and the heart are now formed.

  • There are fingers on the hands
  • The toes are almost formed. There are eyelids over the eyes, but they cannot open yet.
  • Nipples can be seen and the first hair buds form.
  • Muscles begin to form. Early bones are formed. The arms can bend at the elbow.
  • The intestines grow rapidly.
  • The embryo is about 1 ¼ inches long (31 mm).

9-10 weeks from conception (11-12 weeks after the last menstrual period)
By this time, all the main body parts are formed and present. The embryo now is called a "fetus". Growth becomes most important. Fetal length is measured from the top of the head to the curve of the rump (crown-rump) length.

  • The ears move up from around the neck to their normal position.
  • Fetal movements and heartbeat can be seen on ultrasound.
  • Various glands begin to work.
  • The kidneys begin to make urine.
  • The crown-rump length is about 2 1/3 inches (61 mm).
  • The fetus weighs under 1 ounce (14 grams).

11-12 weeks from conception (13-14 weeks after the last menstrual period)
Often, at this point, the sex of the fetus can be seen. The fetus begins to swallow fluid from the amniotic sac (bag of waters). The fluid is replaced with urine made by the kidneys. The placenta is fully formed.

  • Blood cells are made in the bone marrow.
  • The neck can be clearly seen between the head and body.
  • The crown-rump length is about 3 ½ inches (86 mm).
  • The fetus weighs about an ounce and a half (45 grams).

Second Trimester

13-14 weeks from conception (15-16 weeks after the last menstrual period)
The fetal head is still large as the body straightens out. The arms and legs are formed and they can move and bend.

  • Sex organs are almost fully formed.
  • Toenail and fingernail growth begins.
  • The eyes move forward. The ears reach normal position. Now the face is well formed.
  • The crown-rump length is about 4 ¾ inches (120 mm).
  • The fetus weighs about 4 ounces (110 grams).
  • The eyelids close

15-16 weeks from conception (17-18 weeks after the last menstrual period)
Some women begin to feel the first fetal movements, called "quickening." Growth begins to speed up. The legs grow longer, so the fetal head seems less large. Slow fetal eye movements can be seen by ultrasound. The mouth begins to make sucking motions.

  • The bones gain calcium at a rapid rate.
  • The ears stand out from the head.
  • The crown-rump length reaches about 5 ½ inches (140 mm).
  • The fetus weighs about 7 ounces (200 grams).
  • The skin is almost transparent
  • The fetus may sleep and awaken regularly

17-18 weeks from conception (19-20 weeks after the last menstrual period)
Many women feel fetal movement or quickening by this time in pregnancy. The fetal skin is covered by something called vernix caseosa.  Vernix caseosa looks a little like cream cheese. This is about the halfway point of a normal pregnancy.

  • A very fine hair called "lanugo" covers the fetal body.
  • The crown-rump length is about 6 ¼ inches (160 mm).
  • The fetus weighs about 7 ounces (320 grams).

19-20 weeks from conception (21-22 weeks after the last menstrual period)
The skin is red and wrinkled. Blood vessels can be seen very clearly beneath it.

  • Eyebrow and eyelashes start to form.
  • Fingerprints begin to form.
  • The crown-rump length is about 7 ¾ inches (190 mm).
  • The fetus weighs just over 1 pound (460 grams).

21-22 weeks from conception (23-24 weeks after the last menstrual period)
Fetal weight gain is fast during this time. Rapid eye movements can be seen by ultrasound.

  • Lung growth reaches the point where some gas exchange sacs are formed.
  • The heartbeat can be heard with a stethoscope
  • The crown-rump length is 8 ½ inches (210 mm).
  • The fetus weighs 1 pound, 6 ounces (630 grams).
  • At this time, there is a chance the fetus may live if delivered.

23-24 weeks from conception (25-26 weeks after the last menstrual period)
The lungs continue to grow.  The lung cells begin to make a chemical called "surfactant'. Large amounts of surfactant are needed to keep the lungs open between breaths after birth. Fat gradually builds up under the skin.

  • The fetus can suck on fingers or hands.
  • The fetus begins to store fat under the skin.
  • The fetus will blink and act startled in response to loud noises near the woman's belly.
  • The crown-rump length reaches about 9 inches (230 mm).
  • The fetus weighs a little less than 2 pounds (820 grams).

Third Trimester

25-26 weeks from conception (27-28 weeks after the last menstrual period)
The lungs continue to grow. The fetus continues to gain weight. The brain grows and starts to do more complex tasks.

  • Fetal eyes will open slightly.
  • Eyelashes are formed.
  • The crown-rump length reaches about 10 inches (250 mm).
  • The fetus weighs about 2 pounds and 3 ounces (1000 grams).

27-28 weeks from conception (29-30 weeks after the last menstrual period)
The fetal brain can now control body temperature and direct regular breathing. The fetus can weakly grasp at things. Different growth rates from one fetus to another become clear. Some grow more quickly than others.

  • The eyes open wide.
  • Toenails begin to form.
  • Blood cells are made in the bone marrow.
  • The crown-rump length is nearly 11 inches (around 270 mm).
  • The fetus weighs almost 3 pounds (1300 grams).

29-30 weeks from conception (31-32 weeks after the last menstrual period)
More fat builds up under the skin. The skin thickens. The fetus starts to look more like a newborn baby. The lanugo hairs on the face go away.

  • The pupils of the eyes react to light.
  • The fetus may now hiccup
  • The crown-rump length is just over 11 inches (around 280 mm).
  • The fetus weighs about 3 ¾ pounds (around 1700 grams).

31-32 weeks from conception (33-34 weeks after the last menstrual period)
Fat is still building up under the skin as the fetus grows. The lungs keep growing and making more surfactant.

  • The ear holds its shape when moved.
  • Fetal muscle tone increases.
  • The crown-rump length is just under 1 foot (around 300 mm).
  • The fetus weighs over 4 ½ pounds (around 2100 grams).

33-34 weeks from conception (35-36 weeks after the last menstrual period)
The lungs and the nervous system keep growing. Also, more fat builds up under the skin. The fetus begins looking chubby. Hair on the head begins looking normal.

  • Testes in male fetuses start to move from the abdomen into the scrotum.
  • The labia (vaginal lips) in female fetuses begin to cover the clitoris.
  • The fetus moves into a head-down position to prepare for delivery
  • The average crown-rump length is over a foot.
  • The fetus weighs over 5 pounds (around 2500 grams).

35-36 weeks from conception (37-38 weeks after last menstrual period)
In almost all cases, fetal lungs are mature at this point. Lanugo hairs are almost all gone except for the shoulders and upper arms. The fetus may be born now or may stay in the womb while more fat builds up under the skin.

 

37-38 weeks from conception (39-40 weeks after last menstrual period)
This is full term in pregnancy. Most babies are born during this time.

  • The average crown-rump length is over 14 inches (360 mm). The total length counting the legs is about 20 inches.
  • On average, a full-term baby weighs 7 ½ pounds (3400 grams).

References:

  • Cunningham, F.G. Et AL., Williams Obstetrics 21st ed. (2001). Appleton & Lange, Norwalk, Connecticut
  • Guttmacher, A. F. (2003) Dr. Guttmacher's Pregnancy, Birth, and Family planning (2nd ed.). (R. Lichtman, L. L. Simpson, &L.M. Cooper, Eds) New York: New American Library
  • Larsen, W.J., Human Embryology 3rd ed. (2001). Churchill-Livingstone Co., Inc., New York. Moore, K.L. & Persaud Tvn, The Developing Human: Clinically Oriented Embryology 7th ed. (2001). W. B. Saunders Co., Philadelphia.

Acknowledgments
The information on this page was written with the assistance of the following groups:

  • A physician and nurse practitioner review panel
  • American Congress of Obstetricians and Gynecologists
  • Respect Life Office, Diocese of Charleston, Roman Catholic Church
  • Nebraska Department of Health
  • Ohio Department of Health
  • Utah Department of Health
  • Commonwealth of Pennsylvania

Next: Risks of Pregnancy