Foetal Development week by week (beginning with menstruation)

1st Trimester

Weeks 1 – 4

  • The thickened endometrium (lining of the uterus) leaves the body through the vagina
  • The pituitary gland releases FSH (follicle stimulating hormone)
  • The ovaries are stimulated to create several follicles containing immature eggs
  • One follicle will mature into an ovum (egg)
  • The endometrium thickens in preparation for pregnancy
  • The oestrogen levels rise
  • The pituitary gland produces FSH and LH (luteinising hormone)
  • A mature egg ruptures the follicle
  • The egg is released from the ovary
  • The egg travels down the fallopian tube towards the uterus
  • Sperm secretes an enzyme to penetrate the egg
  • Once the sperm and egg fuse the chromosomes combine to form a zygote
  • hCG is released in order to increase progesterone levels and stop menstruation
  • The Zygotes cells divide until there is a cluster of 16 cells (a morula)
  • Fluid collects within the morula creating a separate outer layer which encapsulates the inner cluster of cells
  • The inner layer becomes the embryo; the outer layer becomes the placenta
  • The cluster of cells is now called a blastocyst
  • The blastocyst implants in the uterine wall (implantation)


Week 5

  • The blastocyst is now called an Embryo
  • There are 3 layers to the embryo; the ectoderm (outer layer of skin, central and peripheral nervous systems, eyes, inner ears, and connective tissues); mesoderm (heart, circulatory system, the bones, muscles, kidneys and reproductive system); and the endoderm (a tube lined with mucous membranes for the development of the lungs, intestines and bladder)
  • The embryo is nourished by the yolk sac 

Week 6

  • The brain forms
  • Some cranial nerves are visible
  • The heart which is beating can be seen on an ultrasound
  • The spinal tissue start to form
  • The eyes are beginning to form
  • Arm and leg buds are developing
  • The neural tube begins to close
  • The body has a C-shaped curvature 

Week 7

  • Eye lenses start to form
  • Nostrils become visible
  • The arm buds now look like paddles 

Week 8

  • The lungs begin to develop
  • The ears are forming
  • The eyes are now visible
  • Fingers have begun to form at the ends of the arm buds
  • The spine has started to straighten

Week 9

  • The arms continue to grow and elbows develop
  • The toes start to form
  • All of the essential organs are growing

Week 10

  • The eyelids begin to close
  • The development of the outer ears continues and begins to take shape
  • The intestines rotate
  • The the end of this week the embryo is termed a foetus

Week 11

  • Red blood cells start the form in the liver
  • The eyelids are now fused shut
  • Tooth buds have formed
  • The external genitalia start to develop into a penis or clitoris and labia majora. 

Week 12

  • Finger nails appear
  • The face can now be recognised as a human.

2nd Trimester

Week 13

  • The intestines (which have been growing in the umbilical cord) return to the abdomen
  • Bone tissue is developing
  • The foetus is now urinating into the amniotic fluid

Week 14

  • The ovarian follicles start to form or the prostate appears (depending on whether the foetus is male or female)
  • Red blood cells are forming in the spleen

Week 15

  • The foetus is developing bones
  • The foetus is growing rapidly 

Week 16

  • The movements are becoming coordinated
  • The ears have moved near to their final position
  • The mouth can make sucking motions. 

Week 17

  • Toenails start to form
  • Fat stores begin developing under the skin
  • The skin is transparent
  • Lanugo has appeared on the head 

Week 18

  • The foetus may be able to hear sounds as the ears are now developed

Week 19

  • Vernix starts to cover the foetus 

Week 20

  • This is the halfway point
  • Many women can feel foetal movement by now

Week 21

  • The foetus is able to swallow

Week 22

  • Lanugo completely covers the foetus helping to keep the vernix on the skin
  • The eyebrows and eye lashes appear
  • Meconium is made in the intestinal tract

Week 23

  • The foetus begins to show signs of rapid eye movements
  • Fingerprints are starting to form
  • Taste buds are developing
  • In females the uterus and ovaries which contain a lifetime supply of immature eggs are in place and in males the testes have started to descend from the abdomen 

Week 24

  • Bone marrow begins to make blood cells
  • The foetus has a sleeping and waking pattern
  • Hair has started to grow

Week 25

  • The startle reflex is developing
  • The lower airways of the lungs develop
  • The foetus may respond to sounds by moving

Week 26

  • The lungs are beginning to produce surfactant (which allows the air sacs in the lungs to inflate and deflate)
  • The eyes have finished developing but are still fused shut
  • The foetus may now startle with loud noises

Week 27

  • The lungs are continuing to mature
  • The nervous system continues to develop

3rd Trimester

Week 28

  • The eyelids are partially open
  • Eyelashes have formed
  • The foetus is gaining weight and beginning to look less wrinkled

Week 29

  • The bones are fully developed but they are still very soft

Week 30

  • Red blood cells have formed in the bone marrow
  • The eyes are also open a lot more
  • Hair growth continues
  • The brain is growing rapidly
  • The nervous system is developed enough to control some body functions

Week 31

  • The central nervous system can control body temperature

Week 32

  • The lungs begun to practice breathing
  • Lanugo starts to disappear
  • The foetus begins to absorb minerals (iron and calcium) from the intestinal tract

Week 33

  • The pupils can detect light and they now have the ability to dilate and constrict

Week 34

  • The fingernails have reached the fingertips

Week 35

  • Rapid weight gain continues 

Week 36 

  • The foetus is running out of room to move in the uterus

Week 37

  • The foetus is classed as early term
  • all of the organs are able to function
  • The head may begin its descent into the pelvis 

Week 38

  • The toenails reach the tips of the toes
  • The foetus is developing a firm grasp
  • Most of the lanugo has now disappeared 

Week 39

  • The chest is becoming more prominent
  • The testes continue to descend into the scrotum

Week 40

  • The foetus is considered full term and is ready to be born.



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