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

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.

Eight week old embryo. At this stage of development the embryo looks like a tiny human being.

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 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.

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