fetal development, fetus, crown rump length (CRL), pregnancy week by week, pregnancy weeks

During the first trimester of pregnancy, major changes take place in fetal development. Read about the first 12 weeks of human development, week by week, and see what happens to your baby!

Week 1

  • Fertilized egg begins to divide (cleavage), that is 1 cell divides into 2, which divide into 4,8.16, and so on until a ball of cells (morula) is formed.
  • By the end of the week, this ball has become hollow (a blastula) with a swelled mass of cells on one side (cell mass).  It is at this time that identical twins (monozygotic twins) may form as a result of the blastula dividing completely in two. If the division is not complete, conjoined twins (Siamese twins) develop. These individuals are joined at the spot where the division of the blastula was not completed. 
  • By the 4th or 5th day of development, the developing cell mass or pre-embryo becomes implanted in the lining (implantation). The breakdown of the cells of the uterine lining (endometrium) to form a place for implantation is accomplished by the action of the enzyme hyaluronidase--the same enzyme the sperm uses to get to the egg for fertilization. The nutrients released from these cells are used to provide food for the developing pre-embryo.

Week 2 

  • The two cell layers of the cell mass separate, creating a cavity which will become the amniotic cavity. Cells from the cell mass will form a lining, developing the first of four extra-embryonic membranes, the amnion or amniotic sac, which provides a fluid-filled space that acts as a protective cushion, permits symmetrical, growth, helps the embryo control body temperature, prevents adherence of the embryo to the amnion and protects against infection. The cell mass has become flattened with two cell layers--the ectoderm and the endoderm. It is then called a blastodisc. During gastrulation, a third layer is added between the other two and this is called the mesoderm.  Each layer is destined to form part or all of the organs that will develop during gestation. 
  • The yolk sac is the second extraembryonic membrane to develop. It forms from the endoderm and mesoderm. It will begin making blood until the liver, spleen and then bone marrow takes over the job. An outpocketing of the endoderm of the yolk sac along with some mesoderm forms the 3rd extraembryonic membrane--the allantois a part of which gives rise to the urinary bladder. Mesoderm associated with the allantois combines with the outermost layer of cells around the blastula to form the chorion which will develop the chorionic villi essential to the formation of the placenta.
  • The placenta begins to form (placentation). It is made up of maternal tissue and embryonic tissue. The placenta will be the location of exchange of nutrients and wastes between mother and child.

Week 3 

  •  The embryonic disk becomes more tubular because of differences in the growth rate in different locations. Folding occurs. 
  • The neural groove forms and this will be the site of the development of the spinal cord. One end of the neural groove bends and this will develop into the brain. Pairs of cell clumps (somites) gather on either side of the groove. These will be the site of formation of muscles and vertebrae. 
  • The heart takes shape. It is a tube-like structure with one chamber. Forming blood vessels begin to link up. 
  • At this point, the embryo is between 1.5 and 2.0 mm--about the thickness of 2 dimes.

Week 4

  • During this week things really start to happen quickly!
  • In the heart, differences in wall thickness allow the atria to be distinguished from the ventricles. By day 25, the heart is beating, circulating blood between the embryo, the yolk sac, and the umbilical cord. The umbilical cord is formed by the gathering of the yolk stalk, which connects the embryo to the yolk sac and the body stalk. The body stalk is formed from the chorion and the allantois. 
  • At the center, the neural groove sinks and the sides grow up to begin the formation of the neural tube. Eventually, the sides of the tube will meet and close. It is at this stage that neural tube disorders occur. The outcome of the failure to close is determined by the location at which this occurs (see anencephaly and spina bifida). A region of cells at the top of the neural tube will form the neural crest. These cells will later migrate and contribute to the formation of several structures. 
  • Limbs arise during this week in the form of limb buds. 
  • Folds of the head and face become distinct and spots form marking the beginning of the development of the eyes and ears. 
  • The thyroid, pancreas, liver, kidneys, and lungs are identifiable. 
  • The embryo's sex cells migrate to a location near the developing kidney to form a genital ridge. 
  • By the end of the 4th week, the embryo is curled into an arc about 4mm across--about the size of a small pea.

Week 5

  • Arms and legs get longer and acquire bends where knees and elbows will form tiny paddles develop at the ends of the limbs--these will become hands and feet. 
  • Eyes and ears are distinguishable but the eyes are still very far apart on the face. 
  • The brain continues to develop. Cranial nerves form. The brain curves more due to differential growth rates. 
  • The embryo grows a tail that will later disappear, cartilage begins to form in the head, alongside and underneath the brain. 
  • The development of the embryo kidney continues. 
  • Atria and ventricles begin to divide. The septa between the two atria has a gap in it called the foramen ovale. This does not close until birth and serves to close off circulation to the lungs until that time. There is a flap to prevent the backflow of blood from the left atrium to the right atrium. The atrioventricular septa develop at this time (see Septal Disorders).

Week 6

  • Muscle organization increases.
  • Cartilage begins to form in limb buds; the hands and feet get rays of cartilage that will develop into fingers and toes.
  • By the end of the week, some ossification begins. 
  • The liver grows rapidly creating a bulge near the heart. It takes over the production of blood cells from the yolk sac cells in the male begin to produce testosterone which directs the formation of male reproductive ducts and external genitalia. (If no testosterone is produced, the female structures will develop. Much discussion is going on about the significance of this.) 
  • By the end of this week, the embryo is 15 mm long, about the size of a one peanut shell.

Week 7

  • Spinal nerves begin to form the neural crest cells migrate to form such structures as specialized ganglia, glial cells, cranial and spinal meninges, and spinal nerves that will innervate the limb buds.

Week 8

  • By the end of the 8th week, the face develops an upper lip and nostrils. It is during this time, as the skin folds around the closing parts of the arches to form the top lip and nostrils, the problem of cleft lip can occur.
  • The neck lengthens and makes the jaw stick out a bit. 
  • The ears that were at the level of the neck move to the side of the head and are surrounded by a skin flap. 
  • Eyes are still far apart on the face but have moved forward from their original position and are pigmented. 
  • Internal organs are well defined.
  • Sex organs form around germ cells in the genital ridge. 
  • Muscles in the neck and body contract spontaneously causing movement although the infant is too small for the mother to detect this motion. 
  • Vertebrae enlarge. The only part of the notochord left forms the center part of each intervertebral disc. 
  • Basic heart structure is complete.
  • Cartilage around the brain enlarges. There is more bending of the tissues in the brain which has a dominating cerebral cortex. 
  • The pleural cavity is completely separate from the pericardial cavity and the diaphragm is in place, separating the thoracic and abdominal cavities. 
  • Formation of the adult form of the kidney begins.
  • The 8th week marks the end of the embryonic period--the time in which most errors in development occur. 
  • By the end of this week, the embryo is about 30 mm long--about the size of a two peanut shell.

Weeks 9 & 10

  • The embryo is now called a fetus.
  • The head makes up half of the crown to rump length
  • The neck grows longer and straightens out a little.
  • Eyelids grow over the eyes closing them until the 6th month.
  • Hands and feet are well developed and fingernails and toenails start to form. 
  • While some ossification began earlier, it is during the 9th week that bone formation begins in earnest. 
  • The mandible forms from part of the mandibular arch.
  • Frontal and parietal bones form on the top of the skull. 
  • The maxilla, an important facial bone, begins to form. Ossification centers begin to form the palatine processes which will fuse with the maxilla to form the roof of the mouth. It is at this time that cleft palate may occur. 
  • The diaphragm is completed. 
  • External genitalia begins to develop. Up to this time, it was not possible to distinguish the male fetus from the female fetus externally--both had a creased bump called the phallus. During this week in boys, a burst of testosterone causes the crease to disappear and the phallus to remain. In the weeks that follow, nothing dramatic happens to the female fetus but in the next few weeks the crease stays and the phallus retracts. 
  • Intestines move into the coelom. 
  • The second pharyngeal arch forms the hyoid bone which supports the tongue; arches 3,4, and 6 will form cartilage making up the larynx.

Weeks 11 & 12

  • During week 11 the fetus opens its mouth and will suck a finger if it happens to get in there.
  • By week 12, the fetus begins to practice swallowing by taking in amniotic fluid. 
  • Legs are folded; arms have almost reached their final length.
  • The size of the amniotic sac doubles.
  • Cerebral hemispheres grow over the midbrain. The cerebellum forms. 
  • Ossification of the cranium is well underway. 
  • The gallbladder and pancreas are complete. 
  • The spleen begins blood cell formation. 
  • The mature fetal form of male or female genitalia is in place.
  • Urine formation begins and urine is discharged into the amniotic fluid. 
  • Fetal wastes are transferred across the placenta 
  • By the end of this month, the total length of the fetus is about 87mm.