Fetal development


This article discusses how a human baby is conceived and how the baby develops inside the mother's womb.

See also: Gestational age

Alternative Names

Zygote; Blastocyst; Embryo; Fetus


When sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes.


When a single sperm enters the mother's egg cell, the resulting cell is called a zygote. The zygote contains all of the genetic information (DNA) needed to become a baby. Half of the genetic information comes from the mother’s egg and half from the father’s sperm. The zygote spends the next few days traveling down the Fallopian tube and divides to form a ball of cells.


The zygote continues to divide, creating an inner group of cells with an outer shell. This stage is called a blastocyst. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it.

The blastocyst reaches the womb (uterus) around day 5, and implants into the uterine wall on about day 6. At this point in the mother's menstrual cycle, the lining of the uterus has grown and is ready to support a baby. The blastocyst sticks tightly to the lining, where it receives nourishment via the mother's bloodstream.


The cells of the embryo now multiply and begin to take on specific functions. This process is called differentiation. It leads to the various cell types that make up a human being (such as blood cells, kidney cells, and nerve cells).

There is rapid growth, and the baby's main external features begin to take form. It is during this critical period (most of the first trimester) that the growing baby is most susceptible to damage. The following can interfere with the baby's development:

  • Alcohol, certain prescription and recreational drugs, and other substances that cause birth defects
  • Infection (such as rubella or cytomegalovirus)
  • Nutritional deficiencies
  • X-rays or radiation therapy


The period of time between conception and birth during which the fetus grows and develops inside the mother's womb is called gestation. In humans, the length of pregnancy, or gestational age , is the time measured from the first day of the woman's last menstrual cycle to the current date. It is measured in weeks. Gestational age may also be called menstrual age.

If you got pregnancy using infertilty treatments called assisted reproductive technology, gestational age is determined by adding 2 weeks to the conceptional age.

The following list describes specific changes that occur in the womb:

  • Week 5 of pregnancy (gestational age)
  • Weeks 6 - 7 of pregnancy (gestational age)
  • Week 8 of pregnancy (gestational age)
  • Week 9 of pregnancy (gestational age)
  • Week 10 of pregnancy (gestational age)

The end of the 10th week of pregnancy marks the end of the "embryonic period" and the beginning of the "fetal period."

  • Weeks 11 to 14 of pregnancy (gestational age)
  • Weeks 15 to 18 of pregnancy (gestational age)
  • Weeks 19 to 21 of pregnancy (gestational age)
  • Week 22 of pregnancy (gestational age)
  • Weeks 23 to 25 of pregnancy (gestational age)
  • Week 26 of pregnancy (gestational age)
  • Weeks 27 to 30 of pregnancy (gestational age)
  • Weeks 31 to 34 of pregnancy (gestational age)
  • Week 38 of pregnancy (gestational age)
  • Weeks 39 to 42 of pregnancy (gestational age)


Cunningham FG, Leveno KJ, Bloom SL, et al. Fetal growth and development. In: Cunnigham FG, Leveno KL, Bloom SL, et al, eds. . 23rd ed. New York, NY: McGraw-Hill; 2010:chap 4.

Ross MG, Ervin MG, Novak D. Fetal physiology. In: Gabbe SG, Niebyl JR, Simpson JL, eds. . 5th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2007:chap 2.

Encyclopedia content is provided as information only and not intended to replace the advice and instruction from your personal physician.