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Prenatal development

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Embryogenesis

When sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes. Fertilization usually takes place in the Fallopian tube. A single sperm penetrates the mother's egg cell, and the resulting cell is called a zygote. The term "conception" refers variably to either fertilization or formation of the conceptus (after uterine implantation).

The zygote contains all of the genetic information (DNA) necessary to become a child. 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 called a morula. Further cell division creates 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 uterus at roughly the fifth day. Before embedding into the uterus it first must "hatch" from its glycoprotein shell, known as the zona pellucida. It it then adheres to the uterine lining and becomes embedded in the uterine epithelial cell layer about day six. The inner cell mass further differentiates and is known as an embryo whereas the outer cell layers form embyronic membranes. Together the embryo and the membranes are referred to as a conceptus, or the "products of conception". At this point in the mother's menstrual cycle, the endometrium (lining of the uterus) has grown thicker. The embryo adheres tightly to the endometrium, 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, which produces the varied 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 of differentiation (most of the first trimester) that the growing baby is most susceptible to damage from:

Fetal Development

From the 9th week after fertization until birth (around 38 weeks) the developing human is called a fetus. The fetus is not quite as sensitive as the embryo to damage from environmental exposures. The majority of structures are already formed in the fetus but they continue to grow and gradually become functional.

Changes by weeks of age (and weeks of pregnancy)

The following list describes specific changes in human development by week. "Weeks of pregancy" are dated by obstetricians from the start of the last menstrual period which means that ovulation occurs at the end of the 2nd week.

Pre-implantation

Toxic exposures may cause prenatal death but do not cause developmental defects

  • Week 1 (3rd week of preganancy)
    • Fertilization of the ovum to form a zygote which leads to a blastocyst. Implantation of the blastocyst into the uterus.
  • Week 2 (4th week of pregnancy)
    • Formation of the yolk sac. Cells flatten to form an embryonic disk.
Embryonic Period

Toxic exposures often cause major congenital malformations

  • Week 3 (5th week of pregnancy)
  • Week 4 (6th week of pregnancy)
    • Embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
    • Formation of somites which are the divisions of the future vertebra
    • The heart bulge further develops and begins to beat at a regular rhythm
    • Formation of branchial arches, grooves which will form structures of the face and neck
    • The ear begins to form as otic pits.
    • The brain develops into five areas and some cranial nerves are visible
    • Arm buds and a tail are visible
  • Week 5 (7th week of pregnancy)
    • Embryo measures 8 mm (1/4 inch) in length
    • Lens pits and optic cups form the start of the developing eye.
    • Formation of a primitive mouth and nasal pits.
    • Leg buds form and hands start to form as flat paddles on the arms.
    • Movement of rudimentary blood through primitive vessels connecting to the yolk sac and chorionic membranes
  • Week 6 (8th week of pregnancy)
    • Embryo measures 13 mm (1/3 inch) in length
    • beginning of formation of the lungs
    • further development of the brain
    • arms and legs have lengthened with foot and hand areas distinguishable
    • hands and feet have digits, but may still be webbed
  • Week 7 (9th week of pregnancy)
  • Week 8 (10th week of pregnancy)
    • Embryo measures 30 mm (3/4 inches) in length
    • rotation of intestines
    • facial features continue to develop
    • the eyelids are more developed
    • the external features of the ear begin to take their final shape
Fetal Period

Toxic exposures often cause physiological abnormalities or minor congenital malformation

  • Weeks 9 to 12 (11th to 14th week of pregnancy)
    • the fetus reaches a length of 8 cm (3.2 inches)
    • the head comprises nearly half of the fetus' size
    • the face is well formed
    • eyelids close and will not reopen until about the 28th week
    • tooth buds appear for the baby teeth
    • limbs are long and thin
    • the fetus can make a fist with its fingers
    • genitals appear well differentiated
    • red blood cells are produced in the liver
  • Weeks 13 to 16 (15th to 18th week of pregancy)
    • the fetus reaches a length of about 15cm (6 inches)
    • a fine hair develops on the head called lanugo
    • fetal skin is almost transparent
    • more muscle tissue and bones have developed, and the bones become harder
    • the fetus makes active movements
    • sucking motions are made with the mouth
    • meconium is made in the intestinal tract
    • the liver and pancreas produce their appropriate fluid secretions
  • Week 18 (20th week of pregancy)
    • the fetus reaches a length of 20 cm (8 inches)
    • lanugo hair covers entire body
    • eyebrows and lashes appear
    • nails appear on fingers and toes
    • the fetus is more active with increased muscle development
    • "quickening" usually occurs (the mother can feel the fetus moving)
    • fetal heartbeat can be heard with a stethoscope
  • Week 22 (24th week of pregancy)
    • the fetus reaches a length of 28 cm (11.2) inches
    • the fetus weighs about 1 lb. 10 oz.
    • eyebrows and eyelashes are well formed
    • all the eye components are developed
    • the fetus has a hand and startle reflex
    • footprints and fingerprints forming
    • alveoli (air sacs) forming in lungs
  • Weeks 23 to 26 (25th to 28th week of pregancy)
    • the fetus reaches a length of 38 cm (15 inches)
    • the fetus weighs about 1.2 kg (2 lbs. 11 oz).
    • rapid brain development
    • nervous system developed enough to control some body functions
    • eyelids open and close
    • respiratory system, while immature, has developed to the point where gas exchange is possible
    • a baby born at this time may survive, but the possibilities for complications and death remain high
  • Weeks 27 to 31 (29th to 33rd week of pregancy)
    • the fetus reaches a length of about 38-43 cm (15-17 inches)
    • the fetus weighs about 2 kg (4 lbs. 6 oz)
    • rapid increase in the amount of body fat
    • rhythmic breathing movements occur, but lungs are not fully mature
    • bones are fully developed, but still soft and pliable
    • fetus begins storing iron, calcium, and phosphorus
  • Week 34 (36th week of pregancy)
    • the fetus reaches a length of about 40-48 cm (16-19 inches)
    • the fetus weighs about 2.5 to 3 kg (5 lbs. 12 oz. to 6 lbs. 12 oz.)
    • lanugo begins to disappear
    • increase in body fat
    • fingernails reach the end of the fingertips
    • a baby born at 36 weeks has a high chance of survival, but may require some medical interventions
  • Weeks 35 to 38 (37th to 40th week of preganancy)
    • considered full-term at 37th week of pregnancy
    • may be 48 to 53 cm (19 to 21 inches) in length
    • lanugo is gone except for on the upper arms and shoulders
    • fingernails extend beyond fingertips
    • small breast buds are present on both sexes
    • head hair is now coarse and thicker
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