Fetal Development Week By Week

9 min


The development of the fetus inside your body is an amazingly complex process, one that starts the formation of a zygote. As tiny as a dot, the zygote gradually transforms into a fully developed baby inside the womb. Are you curious about how your baby changes and grows during each week of pregnancy?

If yes, MomJunction details the fetal development stages in each week of pregnancy.

Baby’s Development In The First Trimester


The first trimester of your pregnancy is from weeks 1 to 12. Here is how the baby develops during each week of pregnancy during the first trimester.

Size of the baby

1 to 3 Not measurable during these weeks.
4 Poppy seed 0.04in (0.1cm) <0.035oz (1g)
5 Peppercorn 0.04 in (0.1cm) <0.035oz (1g)
6 Pomegranate seed 0.4in (1cm) <0.035oz (1g)
7 Blueberry 0.4in (1cm) <0.035oz (1g)
8 Cranberry bean 0.62in (1.6cm) (crown to rump) 0.035oz (1g)
9 Cherry 0.9in (0.1cm) 0.07oz (2g)
10 Kumquat 1.22in (3.1cm) 0.14oz (4g)
11 Brussels sprout 1.61in (4.1cm) 0.25oz (7g)
12 Lime 2.13in (5.4cm) 0.5oz (14g)

Week 1 To 3:

At week 1, you have your menstrual flow, which is followed by ovulation in the second week. If the sperm meets the egg within 12 to 24 hours of ovulation, then fertilization happens. The fertilized egg is called a zygote at this stage. It travels through the fallopian tube during the third week and multiplies into a complex cell structure called blastocyst that finally attaches to the uterine wall. Then, conception happens.

Week 4:

  • At week 4, the embryo starts producing the human chorionic gonadotropin (hCG) hormone that helps maintain the endometrium lining.
  • This hormone signals the ovary to stop releasing eggs for the next few months, halting the monthly periods.
  • The embryo has three layers called ectoderm, mesoderm and endoderm, which develop into the tissues and organs of the body.
  • Limb buds and eyes begin to appear.
  • Heart starts beating and the blood vessels start circulation.

Week 5:

  • Face attains a reptilian appearance.
  • Ectoderm develops into the neural tube, which further extends into the brain and spinal cord.
  • The nervous system is developing.
  • The gastrointestinal tract begins to form.
  • Arm and leg buds start to appear with webbed fingers.

Week 6:

Hands and feet have a paddle-like look.
A portion of the brain called cerebral cortex is formed. Synapses, or neurons or nerve cells that communicate with each other, are formed.
The ears begin to take shape, and the mouth develops the salivary glands.
The diaphragm is formed, and a portion of the intestine begins to develop.
Adrenal cortex forms over each kidney, and the pancreas starts producing glucagon.


Week 7:

The facial features such as the eyes, ears, mouth, and nose are distinct by now (10). The umbilical cord that carries oxygen and nutrients for the baby is formed and functional.
The brain divides into three parts, i.e., forebrain, midbrain, and hindbrain. The brain cells are generated at a rate of 100/minute (11).
Skin appears to be thin and translucent.
The nephrons and basic filtration units of the kidneys start to form (12).
The intestine is developing and the digestion process begins.
Liver starts producing red blood cells, and the pancreas starts forming insulin.

Week 8:

The head of the baby is disproportionately large.
The heart develops into four chambers by now.
The spine develops the vertebral column.
Electrical activity starts between the brain and the nervous system.
The umbilical cord continues pumping blood to the embryo.
Eyes are developing with pigmentation of the retina. The intestine starts getting longer.

Week 9:

The eyes and mouth are distinct; the tongue develops taste buds.
Liver starts making the blood cells.
The muscles are forming, allowing slight movement of limbs.
Arms and elbows are developing. The nipples and hair follicles begin to form.
All the essential organs of the body are developing now.
Skeleton starts to form.
The blood vessels are visible beneath the transparent skin.
Fingers and toes are beginning to grow.

Week 10:

In this week, the embryo transitions into a fetus, meaning ‘a young one’ in Latin.

he head is about half the size of the body.
The face attains a human-like appearance.
The bone cells begin to form, along with the skeletal structure.
Fingers and toes develop nails.
Eyelids are developed, and ears start taking shape.

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Week 11:

Hands begin to open and close into fists.
The heart is completely formed and starts pumping blood.
Brain and nervous system are developing.
Fingernails are growing.
Mouth starts developing the teeth buds.
Intestine starts absorbing glucose and water from the swallowed amniotic fluid.
Genitals of the baby can be determined through an ultrasound scan.


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