Fetal Development and Meconium Aspiration | Virtual Anatomy Software

January 11, 2022

Fetal Development and 3D Anatomy

Video explanation of fetal development and meconium aspiration with a patient case example!

In this article, we are going to learn about the stages of fetal development and share a short video on meconium aspiration. Share this article and video with your students!

Embryonic Development: Weeks 1 to 8

When a sperm cell successfully fertilizes an egg, a single-cell structure termed a zygote is formed.  This structure typically has 46 chromosomes - 23 from each parent. These chromosomes are the basic blueprints that will help the zygote develop into an embryo, then a fetus, and eventually a baby. The zygote will undergo rapid cell division as it develops into a morula which, after a few days of continued division, forms a blastocyst during weeks 1-3. As the blastocyst forms it moves from the uterine tubes, where fertilization occurred, into the uterus. When the blastocyst is fully formed it will encounter the endometrium of the uterus, and implantation will occur. Implantation typically occurs within either the fundus or body of the uterus.
Once implanted, the blastocyst begins to form an inner and outer layer of cells. The inner cells become the embryo and the outer layer becomes part of the placenta, which provides important nutrients while the developing baby is inside the womb.

By week 4, the blastocyst is now considered an embryo as the inner cell mass develops into three germ layers. These layers begin to grow and fold, and the circular embryonic disc becomes c-shaped, having formed head and tail folds.  Each layer will differentiate into different organs and structures. The neural tube will become the brain and spinal cord. The heart and other organs are beginning to develop, as well as the limbs and face. By the end of week 8, the embryo is approximately ½ inch long from the crown of the head to the caudal end, and about half the diameter of a US quarter. 


Fetal Development: Weeks 9 to Birth

In the fetal stage, which begins in week 9, the systems of the body develop and mature. The fetus grows in size, from approximately 1/2 inch in length to approximately 20 inches in length as a full-term newborn. Cell growth and maturation continue at a rapid rate as the immature organ systems develop.  

Weeks 9 to 12

During weeks 9 to 12 the fetus is rapidly growing. Some of the noticeable changes that occur are:

  • Anatomical sexual differentiation occurs:
    • Bipotential gonads of the male become the testes and epididymis.
    • Bipotential gonads of the female become the ovaries. 
  • Fetal movements can be felt by the mother and are jerky and frequent. 
  • The liver begins to secrete bile. 
  • Amniotic fluid is circulated by the fetus swallowing it and producing urine. 
  • The eyes are well-developed and the eyelids are fused shut. 
  • Nails begin to develop on the fingers and toes. 

Weeks 13 to 16

During weeks 13 to 16 development continues. Listed below are a few of the developments that occur in this time frame:

  • The eyes move closer together and the eyelids still remain fused shut.
  • The lips start to show sucking motions.
  • The head and ears take on more detailed form as hair begins to grow on the scalp.
  • The urinary and gastrointestinal systems continue to mature. The intestines form meconium, which is fetal feces consisting of ingested amniotic fluid, cellular debris, mucus, and bile

Weeks 16 to 20

The halfway point of a typical pregnancy is approached during this time period. Highlighted below are a few of the developmental changes that occur during this time period:

  • The fetus grows and movements of the fetus become stronger, causing the expecting mother to notice them more easily.
  • The fetus’ sebaceous glands coat the skin with a protective and waxy substance called vernix caseosa, that will moisturize and protect the skin and act as a lubricant during childbirth. 

Weeks 21 to 30

Further development and maturation continue as the fetus grows noticeably in weight during weeks 21 to 30. Listed here are a few of the other notable developments:

  • The fetus experiences rapid weight gain, which will play one of the crucial roles in the regulation of body temperature outside of the womb. 
  • The spinal cord of the fetus has been developing throughout this entire embryonic time, but during these weeks myelination of the axons of the nerve cells begins.

  • Eyelashes grow and the eyelids are no longer fused shut. 

  • The lungs are developing and producing surfactant, which is a substance that reduces surface tension in the lungs.

Weeks 31 to Birth

Leading up to birth, many important final stages of development of the fetus occur. Here a few: 

  • The fetus continues to add fat.
  • The skin goes from red and wrinkly to soft and pink.

Learn more about the details of fetal development and meconium aspiration by watching the BodyViz Brain Builder at the beginning of this article. See 3D visualizations of a fetus growing and a patient case example of a newborn with meconium aspiration!

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Anatomy Terms to Know:

Gamete - germ or sex cell with half the number of chromosomes needed for fertilization.

Zygote - a diploid cell containing 46 chromosomes resulting from the fusion of 2 haploid gametes (the sperm and egg) with 23 chromosomes from each. 

Morula -embryonic stage that consists of 16 blastomere cells compacted into an outer and inner layer.

Blastocyst - a cluster of cells rapidly dividing after fertilization. The outer cells will become the placenta and the inner cells will become the embryo.

Embryo - an early stage of human development from the 4th week of pregnancy to the end of the 8th week. 

Fetus - the later stage of human development from the 9th week of pregnancy to birth.

Bipotential Gonads - undifferentiated gonads that can develop into male or female sexual organs.

Surfactant - a substance that reduces surface tension in the lungs and helps the lungs to expand.

Meconium - the first feces of a fetus or newborn composed of amniotic fluid, mucus, bile, hair, as well as skin and intestinal epithelial cells.

Vernix Caseosa - a white, creamy film that develops and covers a fetus to provide protection and moisture to the skin and also can act as a lubricant during delivery. 


  1. What are the time frames and developmental order of the fetus, blastocyst, and embryo stages?
    A: Weeks 1-3: Blastocyst, Weeks 4-8: Embryo, Weeks 9-Birth: Fetus

  2. How many chromosomes are in a typical zygote, sperm, and egg cell?
    A: Zygote: 46; Sperm: 23; Egg: 23

  3. What is implantation and where does it normally occur?
    A: Implantation is the process of the blastocyst burrowing into the uterine lining and is where the embryo will begin to develop.

  4. What do the bipotential gonads of males and females develop into?
    A:  Females: ovaries, Males: testes and epididymis. 

  5. Surfactant reduces surface tension in what organ(s)?
    A: The lungs

  6. What four-week period does meconium begin to develop in the intestines?
    A: Weeks 13-16

  7. What 3 purposes does the vernix caseosa have?
    A: Lubrication, moisturization, and protection

  8. What does the inner layer of the blastocyst develop into?
    A: The embryo
  9. What does the outer layer of the blastocyst develop into?
    A: Part of the placenta

  10. During what four-week period do the gonads of the male and females begin to form?
    A: Sexual differentiation begins during weeks 9-12

Want a free ready-to-use worksheet you can assign to students?

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Pregnancy Week by Week - Mayo Clinic

Meconium Aspiration Syndrome - John Hopkins

Fetal Development 28.3 - Anatomy and Physiology - Open Stax

Meconium Aspiration Syndrome - Merck Manual

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