Fetal development is the term used to refer to human  development before birth.  Human  development begins at conception or fertilization and continues throughout the  individual’s lifetime.  Conception or  fertilization refers to “the process by which the male’s sperm unites with the  female’s ovum.  By this event, a new life  is created and the sex and other biological traits are determined.  These traits are determined by the combined  genes and chromosomes that exist in the sperm and ovum.”1

The new life that begins at conception/fertilization is  referred to by different terms depending on the stage of its development.  From conception until its implantation into  the uterus, the new life is referred to as the zygote.  Once implantation occurs between 7-10 days  after conception, the growing life is known as the embryo.  The embryonic stage of development continues  until week eight, at which point all the organ systems are present, the heart  is fully developed and the embryo looks like a tiny human being.  From week eight until birth, the life in the  womb is referred to as the fetus.2  During the fetal phase, the organ systems  become more mature, and the fetus grows from approximately 3cm and 2.5g at 60  days to 50 cm and 3300g at term.3

During the zygotic and embryonic stages, development occurs  by cell division and specialization into various organ systems.  The fetal phase is characterized by  maturation of the existing organs and immense fetal growth. 4

Life Begins (Weeks 1  and 2)

The moment the ovum and sperm join at  conception/fertilization, the life of a new, distinct, human individual  begins.  Fertilization of the ovum  usually occurs as it travels from the female ovary through the uterine tube,  and into the body of the uterus.  When  the sperm and ovum unite, the resulting zygote develops from one cell into  about 32 cells by day four after fertilization.   By this time, the cells start to specialize, either into embryonic  cells, or cells that will form the placenta and the membranes that will  surround the embryo.  Within 72 hours of  fertilization, the zygote reaches the uterus where it will implant 7-10 days  after fertilization.5  By the end of the second week, implantation  is complete.6,7

As implantation occurs, the placenta starts to form.  The placenta is an essential, though  temporary organ, which serves as a vital pathway between the mother and  fetus.  It transfers oxygen from mother  to fetus, and carbon dioxide from fetus to mother.  It transports nutrients from mother to fetus,  and wastes from fetus to mother.  It  transfers heat from mother to fetus, and helps to regulate the effect that  maternal drugs and medications have on the fetus.  It produces hormones essential for fetal  growth and for the continuation of the pregnancy.8, 9

Week 3
During the third week, the embryo’s heart and circulatory  system,10 kidneys,11 brain, spinal cord, gastrointestinal tract and diaphragm12 start to develop.  Leg and arm buds begin  to form13 and the embryo is approximately 2 mm long from crown to rump.14

Week 4
By week four, the embryo’s leg and arm buds become more  noticeable.  The lungs begin formation;15 the liver and pancreas appear as buds16 and a “single-tubed heart begins to beat at 21 – 25 days from fertilization.”17 By twenty-eight days’ gestation, the heart is able to circulate its own blood.18

Week 5
By the fifth week, the leg and arms buds are more fully  developed.19   The eyes and ears begin to form and nerves  become visible in the brain.20  It is during the fifth week that the brain, heart  and circulatory system will “show the most advanced development.”21

Week 6
The sixth week involves development of the external ears,22 primitive skeleton and central nervous system.   Brain waves can be detected, and the liver begins to produce red blood  cells.23  At this stage “the upper and lower jaws are  recognizable … the upper lip has formed, and the palate is developing.”24

7 week embryo

Seven week old embryo

Week 7
During week seven, the arms and legs begin to move, elbows  and toes are visible and the mouth and lips become visible.  Teeth buds, hair follicles, the diaphragm25 and nipples begin to form.26  The eyelids and tongue are also starting to  develop.27  Towards the end of the seventh week,  cartilage will begin to develop, and this cartilage will soon start converting  into bone.28
Week 8
By week eight, the embryo’s heart is complete and facial  features continue to develop.29  External genitals begin to form according to  the gender determined at conception.   During the eighth week, the larger muscles are able to contract and bones  start to form.  By the end of this week,  all the major organ systems are present,30 the embryo “clearly resembles a human being” and is approximately 3 cm long  from the crown of the head to the rump.31  This marks the end of the embryonic stage of  development.
web_8 wk preborn babyEight week old embryo.
At this stage of development
the embryo looks like a tiny human being.

11 weeks_feet






11 week old fetus
The toes are almost completely formed.

Week 9 – 12
The beginning of week nine signals the start of the fetal  stage of development.  Between the ninth  and twelfth week of development the head will account for almost half the size  of the fetus.  The digestive system  begins to function, urine is produced and excreted, blood starts to form in the  bone marrow,32 vocal cords begin to form allowing the fetus to make sounds33 and the face, neck, fingers and toes are almost completely formed.  By twelve weeks’ gestation the fetus can make  a fist, weighs approximately 45 grams, is 5 cm long from crown to rump and has  a heart beat that can be detected electronically.34  At this point in development the gender of  the fetus can also be determined visually.35
Week 13 – 16
During week thirteen to sixteen, fine hair develops on the  head, bones become harder, the fetus starts to make sucking motions, the skin  is transparent, and fingernails are detected.   By week fifteen, the sensory system begins to function, allowing the  fetus to sense pressure and touch, as well as feel pain.36  The fetus will also gain 4 times its weight  during these four weeks.37  The mother may be able to feel fetal movement  by the sixteenth week.38  By the end of the sixteenth week the fetus  will have a functioning pancreas and liver and will also be able to swallow  amniotic fluid.39

fine hair
16 week old fetus
Facial features are clearly distinguishable


web_baby20 weeks old fetus

Week 17 – 20
By the seventeenth and eighteenth week of development, the  heartbeat can be heard with a stethoscope.   The eyebrows and head hair appear, the muscles are well developed40 and the fetus can produce antibodies.  By  the end of the twentieth week the fetus will have developed “definite sleeping  and activity patterns … that will guide sleep/wake patterns throughout life.”41  By the twentieth week, the fetus weighs 435 –  465 grams and is approximately 19 cm long.42

Week 21 – 24
Between weeks twenty-one and twenty-four, the eyelashes and  eyebrows are almost fully formed, and the pupils react to light.  The fetus responds to loud or sudden sounds  with a startle reflex and can grasp with his/her hands.43  The formation of blood in the bone marrow  increases as formation of blood decreases in the liver.44  At this stage in development the fetus has  reached viability, which means that it has the chance of surviving outside the  womb.45  At twenty-four weeks’ gestation the fetus  weighs approximately 780 grams and measures 24 cm from crown to rump.


Week 25 – 28
During weeks twenty-five to twenty-eight, the formation of  blood cells completely transfers to the bone marrow, the central nervous system  controls some functioning, the eyelids can open and close and the fetus has its  own distinct fingerprints.  The fetus is  now approx. 28 cm long, weighs about 1200-1250 grams and usually moves into a  head-down position to prepare for birth.

090413185734-large27 week fetus 3D ultrasound image

Week 29 – 32
Between the twenty-ninth and thirty-second week, the central  nervous system takes more control of body functions and the rhythmic movement  of breathing begins.  The lungs are not  yet fully mature at this stage.  By the  end of the thirty-second week, the fetus responds to sounds from the outside  world.46  Body fat rapidly increases, and by the end of  week thirty-two, the fetus weighs almost 2000 grams47 and measures 30 cm.48

Week 33 – Term
From week thirty-three until birth, body fat continues to  increase, fine hair starts to disappear and breast buds form on males and  females.  The mother supplies the fetus  with antibodies against disease and the fetus completely fills the uterus.  The final weeks of development are committed  to the maturation and growth of the fetus.   During the final weeks, the fetus gains approximately 14 grams of fat a  day.49  All body systems are functioning, and the  fetus is ready to meet the world outside the mother’s womb.  Given oxygen, nutrition, and a nurturing  environment, the healthy fetus will continue to develop into an infant,  toddler, adolescent, adult, and senior, until death.


1 Miller  BF, et al.  Encyclopedia and Dictionary of Medical,  Nursing and Allied Health, 5th Edition.  W. B. Saunders Company, 1992:547.

2 Pillitteri  P.  “Maternal & child health nursing:  care of the childbearing & childrearing family – 5th  edition.”  Lippincott Williams &  Wilkins: Philadelphia,  2007: 183.

3 Seeley  RR, et al.  “Anatomy and Physiology, 2nd Edition.  Mosby.”  Year Book Inc: St Louis Missouri,  1992: 948-66.

4 Gardosi  J.  “Normal fetal growth,” in Dewhurst’s textbook of obstetrics and  gynaecology – 7th edition.   Edited by D. Keith Edmonds.   Blackwell Publishing: Massachusetts,  2007: 30.

5 Pillitteri.  p. 184.

6 Ricci  SS.  “Essentials of maternity, newborn  and women’s health nursing.”  Lippincott  Williams & Wilkins: Philadelphia,  2007: 212.

7 Vidhya  T, et al. “Normal embryonic and fetal  development,” in Clinical Obstetrics: the  fetus & mother handbook.  Edited  by E. Albert Reece, et al.  Blackwell Publishing Ltd.: Massachusetts, 2007.

8 Lotas,  MJ, et al.  “Critical Periods in Development” in Developmental care of newborns &  infants: a guide for health professionals.   Edited by Carole Kenner, et al.  Mosby: Philadelphia,  2004: 90.

9 Reeder,  S., et al.  “Maternity Nursing: Family, Newborn and  Women’s Health Care,17th editon.”   J.B. Lippincott company, 1992:155-162.

10 Vidhya. et al.

11 New    Jersey, 2004:239

12 Seeley. et al. p. 958-959.

13 Ricci. 316.

14 Olds. et al.   239.

15 Vidhya. et al.

16 Seeley. et al. p. 958-959

17 Seeley. et al. p.958-959.

18 Olds. et al. p. 239

19 Olds, et al. p. 239

20 Ricci.  p. 216.

21 Olds. et al. p. 242.

22 Jones  RE.  “Human reproductive biology – 3rd  edition.”  Elsevier Inc.: Massachusetts, 2006:  10.

23 Ricci.  p. 216.

24 Olds, et al. p. 239.

25 “Midwifery by ten teachers.”  Edited by  Debbie Holmes, et al.  Oxford  University Press: New York, 2006: 21.

26 Ricci.  p. 216.

27 Olds, et al.   p. 239.

28 Cronin  A, et al. “Human Development and  Performance Throughout the Lifespan, 1st edition.”  Thomson Delmar Learning: New York, 2005: 105.

29 Ricci.  p. 216.

30 Seeley. et al. p. 948.

31 Olds. et al. p. 242.

32 “Maternity Nursing, 7th edition.”  Edited by Lowdermilk DL, et al.  Mosby Inc.: Missouri, 2006: 202.

33 Lotas, et al. p. 99.

34 Olds. et al. p. 243.

35 Ricci.  p. 216.

36 Lotas. et al. p. 99.

37 Ricci.  p. 216.

38 Cronin. et al. p. 108.

39 Pillitteri. p. 196.

40 Ricci.  p. 216.

41 Pillitteri. p. 196.

42 Olds. et al. p. 243.

43 Pillitteri. p. 196.

44 “Maternity Nursing.” p. 203.

45 Cronin. et al. p. 108.

46 Pillitteri. p. 196.

47 Olds. et al. p. 244.

48  Cronin. et al. p. 96.

49 Cronin. et al. p. 110.