Fetal Period

The fetal period is the period of prenatal development that begins eight weeks after fertilization and ends at birth. Thus it includes the end of the third trimester, and the whole of the second and third trimesters. During this time, the fetus grows from about 3 grams to more than 3 kilograms on average, and the organs that appeared in the preceding embryonic period develop further, structurally and functionally.

Human fetus about eight and a half weeks old. Gray's Anatomy (1918) figure 63 (originally from His). Source: WikiMedia Commons

Fetus in utero, between fifth and sixth months. Gray's Anatomy (1918) figure 38. Source: WikiMedia Commons

Size and Appearance

The fetus grows dramatically in length and weight during the fetal period. The rest of the body grows more rapidly than the head, so that the relative size of the head shrinks during the fetal period from roughly ½ to ¼ of the crown-heel length.

Table. Mean measurements of length and weight during the fetal period

The mean measurements presented here are from several tables and figures in O'Rahilly and Müller^[1]

Organ development

The fetal period is also characterized by growth and maturation of tissues and organs that formed in the embryonic period. The milestones of the fetal period include the following:

• Heart sounds can be detected as early as 8 or 9 weeks post fertilization (p.f.) with a fetal Doppler ultrasound stethoscope, and 20 weeks p.f. with a stethoscope or fetoscope. (An external electronic fetal monitor can be used to obtain fetal heart rate in the third trimester in antepartum testing and intrapartum monitoring.)

• Ossification begins at the end of the embryonic period. Most primary ossification centers of skull, long bones and vertebrae appear by 12 weeks p.f.. The epiphyses of long bones are still cartilaginous at birth.

• The external genitalia sexually differentiate during weeks 9-12 p.f..

• Mother first feels fetal movements at about 16-20 weeks p.f. This is the phenomenon known as "quickening".

• The lungs become capable of breathing air at the beginning of the 3rd trimester (about 25-26 weeks p.f.) when the lungs enter the saccular phase of development and begin to produce surfactant. Lung maturity is the primary determinant of viability.

• Hematopoiesis begins in the bone marrow in the second trimester, and in the third trimester bone marrow replaces the liver and spleen as the primary site of hematopoiesis.

Viability

The age of viability is about 20-26 weeks p.f. (22-28 menstrual weeks), and is subject to a number of factors.

Biological considerations

• A fetus may be considered viable as early as 20 weeks post-fertilization; however, the survival rate after birth is low.
  

• The survival rate is also dependent on birth weight and access to special care. Extremely low birth weight babies (<500 g) have a low survival rate, even with high tech medical care because the lungs are immature. Premature babies with low birth weight (1500-2500 g) have a low mortality rate but a high morbidity rate.
  

• The primary determinant of viability is lung maturation. The lungs usually become capable of breathing air at the beginning of the 3rd trimester (about 25-26 weeks) when the lungs enter the saccular phase of development and begin to produce pulmonary surfactant. Infant respiratory distress syndrome (hyaline membrane disease) in premature infants may be prevented by giving glucocorticosteroids to mothers about to deliver prematurely, to stimulate production of surfactant.
  

Legal Definitions

• Age of viability is the age at which a fetus is "potentially able to live outside the mother’s womb [that is, can survive], albeit with artificial help" (Roe v. Wade, US Supreme Court, 1973). In the US and Canada, the courts have generally used 22 weeks p.f. (i.e., 24 weeks of gestation) as recommended by North American medical societies. Prior to the development of advanced modern medical care, the age of viability was considered to be the beginning of the third trimester.
  

• Currently, the World Health Organization recognizes the age of viability to be 20-26 weeks p.f. (22-28 weeks of gestation), depending on local availability of advanced medical care.^[2] But for the purposes of compiling health statistics, the WHO defines viability as "fetuses weighing at least 500 g (or, when birth weight is unavailable, after 22 completed weeks of gestation or with a crown-heel length of 25 cm or more)."^[3]

Embryonic or Fetal Loss

• Embryonic or fetal loss before viability is termed a miscarriage or spontaneous abortion. Spontaneous abortion is very common (greater than 50% of pregnancies), especially in the first four weeks post-fertilization.

• Fetal loss after viability is termed stillbirth. Stillbirths account for 0.5-1% of births.

References

1. ↑ O'Rahilly R and Müller F. Human Embryology & Teratology, 3rd Ed. New York: Wiley-Liss, 2001.
2. ↑ World Health Organization. Basic Newborn Resuscitation. WHO, 1997. full text
3. ↑ World Health Organization. Reproductive Health Indicators: Reproductive Health and Research Guidelines for their generation, interpretation and analysis for global monitoring. WHO, 2006. pdf