Timeline of Prenatal Development

Elise Wantling and Allison Blevins

1 Week Before Fertilization


  • ovulation occurs: one egg erupts from its follicle and travels into the uterine tube
  • increase in estrogen and progesterone lead uterus to form a thick lining to support a fertilized egg

Fertilization Occurs

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1-4 Days After Fertilization


  • the fertilized egg has become a zygote, having divided into 16 identical cells called cleavage
  • the zygote enters the uterus and is called a morula

5 Days After Fertilization

  • implantation occurs: the morula begins to burrow into the lining of the uterus
  • it is now called a blastocyst
  • the blastocyst has an inner cell mass (will become the embryo), a fluid-filled cavity (will become the amniotic sac), and an outer cell mass (will become the placenta)

1 Week After Fertilization

  • the blastocyst cells have been multiplying
  • the part that will become the placenta is producing the hormone hCG (human chorionic gonadotropin) which signals the ovaries to stop releasing eggs, and triggers increase of estrogen and progesterone which keep the uterus from shedding its lining
  • amniotic fluid is beginning to collect
  • blastocyst is exchanging nutrients and waste through a primitive circulatory system that connect the body to the lining of the uterus

2 Weeks After Fertilization

  • blastocyst is the size of a poppy seed
  • organs begin to form
  • the blastocyst consists of two layers: epiblast and hypoblast
  • placenta forms and begins to tunnel into the lining of the uterus to provide nutrients for the blastocyst
  • amniotic sac forms
  • yolk sac produces the baby’s red blood cells and helps deliver nutrients until the placenta is developed enough to take over
  • gastrulation occurs

3 Weeks After Fertilization

  • the blastocyst, now called a gastrula, is about the size of a sesame seed
  • it is now made up of three layers: endoderm, mesoderm, and ectoderm
  • the neural tube (which will from the nervous system) is starting to develop in ectoderm
  • the heart and circulatory system begin to form in mesoderm
  • the heart starts to divide into chambers and beat
  • primitive placenta and new umbilical cord provide nutrients

4 Weeks After Fertilization

  • the gastrula is about the size of a lentil
  • the mouth, nose, and ears start to form
  • intestines are developing
  • the pituitary gland and the rest of the brain are forming
  • muscles and cartilaginous bones begin to form

5 Weeks After Fertilization

  • the embryo is about the size of blueberry
  • arms and legs are developing
  • hands and feet begin to form
  • eyelids begin to form
  • eyes have some color
  • brain is growing
  • liver is operating, appendix and pancreas are formed

6 Weeks After Fertilization

  • the embryo is about the size of a kidney bean
  • webbed fingers and toes begin to form
  • breathing tubes have formed and connect to the still-developing lungs
  • brain cells are forming primitive neural pathways
  • the first brain waves can be detected

7 Weeks After Fertilization

  • the embryo is about the size of a grape
  • the heart has divided into four chambers and valves start to form
  • teeth begin to form
  • organs, muscles, and nerves start to operate
  • external sex organs have formed but are not distinguishable yet
  • sensory organs have become more distinct
  • the placenta begins producing hormones

8 Weeks After Fertilization

  • the embryo is now a fetus
  • the fetus is now the size of a kumquat
  • vital organs are now beginning to function
  • the fetus has thin hair at this point

9 Weeks After Fertilization

  • the fetus has grown to size of a fig
  • bones are beginning to harden
  • the diaphragm has formed and the fetus can hiccup

10 Weeks After Fertilization

  • the fetus is the size of a lime
  • the fetus develops reflexes this week
  • nerve cells are multiplying rapidly

11 Weeks After Fertilization

  • the fetus is now the size of a pea pod
  • the fetus has fingerprints
  • the internal sex organs have developed, and female fetuses will have more than 2 million eggs in their ovaries

The Second Trimester

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12 Weeks After Fertilization

  • the fetus has grown to the size of a lemon
  • the kidneys begin producing urine
  • the liver begins producing bile
  • the spleen starts producing red blood cells

13 Weeks After Fertilization

  • the fetus is the size of an apple
  • the ability to sense light develops
  • taste buds are forming
  • the external sex organs are now distinguishable from each other - the biological sex of the fetus can be determined through an ultrasound

14 Weeks After Fertilization

  • the fetus is the size of an avocado now
  • sensory organs have begun to move to their final position
  • toenails start growing

15 Weeks After Fertilization

  • the fetus is about the size of a turnip
  • the cartilage skeleton begins to ossificate (turn to bone)
  • the joints are flexible
  • sweat glands start to develop

16 Weeks After Fertilization

  • the fetus is now the size of a bell pepper
  • myelin is beginning to form around the nerve cells

17 Weeks After Fertilization

  • the fetus has grown to the size of an heirloom tomato
  • hearing has developed
  • a waxy coating has covered the skin to protect it from the amniotic fluid

18 Weeks After Fertilization

  • the fetus is the size of a banana

19 Weeks After Fertilization

  • the fetus is about the size of a carrot
  • eyebrows have formed
  • eyelids have finished forming
  • in female fetuses, the vagina begins to develop

20 Weeks After Fertilization

  • the fetus is around the size of a spaghetti squash
  • lips, eyelids, and eyebrows are more distinct

21 Weeks After Fertilization

  • the fetus is now the size of a large mango
  • the sense of movement has developed
  • some sounds become familiar to the fetus

22 Weeks After Fertilization

  • the fetus is the size of an ear of corn
  • the respiratory system is getting ready to breathe once the baby is born

23 Weeks After Fertilization

  • the fetus is the size of a rutabaga
  • the fetus is starting to gain fat, filling out the previously wrinkled skin

24 Weeks After Fertilization

  • apparently at this stage, the fetus is approximately the size of a bunch of scallions
  • it is now inhaling and exhaling amniotic fluid

25 Weeks After Fertilization

  • the fetus is the size of a head of cauliflower
  • the fetus sleeps and wakes regularly
  • the brain is very active

Third Trimester

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26 Weeks After Fertilization

  • the fetus is the size of a large eggplant
  • eyelashes have grown in
  • the fetus can now blink

27-31 Weeks After Fertilization

  • the fetus spends much of this time growing and gaining fat
  • by the end of this period, the fetus will be the size of a pineapple

32 Weeks After Fertilization

  • the fetus is the size of a cantaloupe
  • the central nervous system is maturing
  • the lungs are developing further

33 Weeks After Fertilization

  • the fetus is the size of a honeydew melon
  • the kidneys have fully developed
  • the liver can break down some waste

34-36 Weeks After Fertilization

  • the waxy protective coating covering the fetus' skin is gone
  • the lungs and brain fully mature
  • by the end of week 36, the fetus will be about the size of a leek

Full Term: The Baby Can Now Be Born Safely!

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