The Prenatal Developmental Period

  • The human body, like that of most animals, develops from a single cell produced by the union of a male and a female gamete (or sex cell). 
  • Cell division, cell migration, programmed cell death (apoptosis), differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell, a zygote, into a multicellular human being.
  • It is customary to divide human development into prenatal (before birth) and postnatal (after birth) periods.
  • Most changes occur during the prenatal period; however, important changes also occur during later periods of development:
    • neonatal period (first 4 weeks)
    • infancy (first year)
    • childhood (2 years to puberty)
    • adolescence (11 to 19 years).
  • Development does not stop at birth; other changes, in addition to growth, occur after birth (e.g., development of teeth and female breasts).

The Prenatal Developmental Period

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The Prenatal Developmental Period

  • Prenatal development (from Latin natalis, meaning ‘relating to birth’) includes the development of the embryo and of the fetus during a viviparous animal’s gestation.
  • In human pregnancy, prenatal development is also called antenatal development.
  • Prenatal development is highly influenced by the inheritance, expression, and regulation of genes.
  • The development of a human from fertilization of an oocyte to birth is divided into two main periods:

(a) Embryonic period and

(b) Fetal period.

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The Embryonic Period

  • The embryonic period extends from fertilization to the end of eight weeks and the developing organism is called an embryo.
  • The embryonic period is further divided into two parts:

(a)  pre-embryonic period (germinal stage) and

(b) embryonic period proper.

Pre-embryonic period:

It extends from conception (fertilization) to the end of the second week of intrauterine life (IUL).

The morphogenic events during this period include:

  • fertilization
  • transportation of zygote through the uterine tube
  • mitotic divisions/cleavage
  • implantation, and
  • formation of primordial embryonic tissues.

Embryonic proper period:

  • It extends from the beginning of the third week to the end of the eighth week of intrauterine life.
  • It is also called the period of organogenesis.

The morphogenic events during this period include:

  • differentiation of the germ layers into specific body organs
  • formation of the placenta, umbilical cord, and extraembryonic membranes.

The Fetal Period

  • The fetal period extends from the beginning of the ninth week (third month) until the birth
  • During this period, there is tremendous growth and specialization of the body structures.
  • It is characterized by the maturation of tissues and organs and the rapid growth of the body.
  • Growth in length is particularly striking during the third, fourth, and fifth months, while an increase in weight is most striking during the last 2 months of gestation.

The prenatal period ends with parturition and is followed by a long postnatal period. Only at about age 25 years are the last progressive changes completed.

References

  1. Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R., Francis-West, P.H. & Philippa H. (2015). Larsen’s human embryology (5th ed.). New York; Edinburgh: Churchill Livingstone.
  2. Sadler, T. W., & Langman, J. (2004). Langman’s medical embryology. Philadelphia, Pa: Lippincott Williams & Wilkins.
  3. Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2008). The developing human: Clinically oriented embryology. Philadelphia, PA: Saunders/Elsevier.
  4. Gilbert, S. F. (2000). Developmental biology. Sunderland, Mass: Sinauer Associates.

About Author

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Yashaswi Sharma

Yashaswi Sharma did her bachelor's in Microbiology from St. Xavier’s College, Kathmandu, Nepal, and completed her M.Sc. in Reproductive Biology and Clinical Embryology from AIIMS (All India Institute of Medical Sciences), New Delhi, India. She is currently doing her Ph.D. from the Polish Academy of Sciences. Her field of interest is Scientific Research, Obesity Research, Assisted Reproduction and Embryology.

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