Foetal Development – An Overview

There are three trimesters in pregnancy and each has different developmental characteristics. The estimated due date (EDD) is set by calculating 40 weeks from the first day of the last monthly period (LMP); which means that when fertilisation occurs the woman is already two weeks pregnant.

For a detailed view of foetal development week by week – with pictures! Please click here.

In the first trimester, when an ovum (egg) is released it travels down the fallopian tube towards the uterus; the corona radiata (outer layer of the ovum) contains follicular cells that are difficult to penetrate; spermatozoa (sperm) must then secrete a digestive enzyme in order to weaken the corona radiata. Once the sperm have penetrated the egg the chromosomes combine to form a zygote (a one-celled body which contains 46 chromosomes); hCG is then released to increase progesterone levels and stop your period. The zygotes’ cells divide rapidly 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 will eventually become the embryo and the outer layer will form the placenta; there are now 58 cells in the structure and it is now called a blastocyst.

The blastocyst then burrows into the uterine wall (implantation). During the embryonic period (weeks five to ten) the major structures begin development; there are three layers to the embryo; the outer layer (ectoderm) forms the outer layer of skin, nervous systems, eyes, inner ears, and connective tissues; the middle layer (mesoderm) forms the heart and circulatory system, along with the bones, muscles, kidneys and the reproductive system; and the inner layer (endoderm) becomes a tube lined with mucous membranes ready for the development of the lungs, intestines and bladder.

While the placenta is forming the embryo is nourished by the yolk sac. The brain forms and the heart is starting to pump blood through the main blood vessels; the tissue which will become the spine is growing and has developed somites; the eyes are beginning to form, and arm and leg buds are developing. The neural tube then closes and the ears and nostrils begin to develop; the lungs are also forming. By the seventh week the arm buds look like paddles which will develop into fingers. The spine eventually begins to straighten and as the arms continue to grow they can bend at the elbows; toes then start to form and all of the essential organs have begun to grow. The eyelids then fuse shut, and the intestines rotate. At the end of week ten the embryo is termed a foetus. Red blood cells begin to form in the liver, tooth buds appear and the external genitalia starts to develop into either a penis or clitoris. By the end of this trimester the embryo has the appearance of a miniature human.

During the second trimester the intestines (which have been growing in the umbilical cord) return to the abdomen and bone tissue is developing. The ovarian follicles start to form in females and the prostate appears in males. Red blood cells are developing in the spleen and bones have also begun developing, with movements becoming better coordinated; the ears move near to their final position and the foetus can hear sounds outside of the womb; the mouth now makes sucking motions. Fat stores form under the skin, and vernix (a greasy coating) covers the foetus offering protection from abrasions and chapping due to the amniotic fluid. The next stage of development is the swallowing reflex; the foetus swallow’s amniotic fluid and then urinates before swallowing again, this helps to mature the lungs; lanugo (soft, fine hair) covers the foetus helping to keep the vernix in place; and meconium is now made in the intestinal tract. The foetus begins to show signs of rapid eye movements, fingerprints are starting to form and 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. Bone marrow is now making blood cells and the startle reflex is developing; the foetus may respond to sounds with movement. Surfactant is now being produced in the lungs, allowing the air sacs to inflate and deflate. 

In the third trimester the foetus rapidly gains weight; the bones are fully developed although still soft, and red blood cells have formed in the bone marrow. The eyes open and the pupils can now detect light; the brain is growing rapidly and during the thirty-first week the central nervous system can control body temperature. The lungs begin to practice breathing, lanugo starts to disappear and the foetus begins to absorb minerals (iron and calcium) from the intestinal tract. At thirty-seven weeks the foetus is classed as early term, all organs are able to function and the head may begin its descent into the pelvis. The foetus is considered full term at 40 weeks.

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.