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Prenatal Form and Function – The Making of an Earth Suit


Unit 4:   3 to 4 Weeks

  Closer Look: 
  Applying the Science: 

Vital Systems – The Beating Heart

Only 3 weeks and 1 day after fertilization - the heart begins to beat.1 By 4 weeks, the heart typically beats between 105 and 121 times per minute.2

The Human Heart
Figure 4.1 - The Human Heart
Copyright © 2002 Lippincott, Williams & Wilkins.
All rights reserved.
embryonic heartbeat
Figure 4.2 - Heartbeat
Red blood cells flow through early blood vessels in the embryo’s first working body system.
The Biology of Prenatal Development DVD
Copyright © 2006 EHD, Inc. All rights reserved.

Hearing the Fetal Heartbeat


Doctors use several different methods to listen to the fetal heartbeat. About 3 weeks, one day after fertilization, when the heart first begins to beat, the sound of the little heart is too soft to hear. Very soon thereafter, they can see the motion using ultrasound technology.

A doctor's stethoscope is not sensitive enough to hear the embryo's heart beating.

A special stethoscope called a fetoscope works well when the fetus is larger, usually around 15-17 weeks.1 An active fetus however, can make this method of listening a bit challenging. Often when the fetoscope is finally in the right place on the woman's belly, her fetus will change positions and the doctor must move the stethoscope again in search of the fetal heartbeat.

Doctor, pregnant woman

The presence of a fetal heartbeat confirms pregnancy, as long as doctors are certain to distinguish the fetal heartbeat from the mother's. Usually this is not difficult as the fetus has a much faster heart rate than the mother. Sometimes during an examination, a woman's heart rate will increase and actually cause the fetal heart to beat more rapidly.

Noises from within can make the mother's and fetal heartbeat hard to distinguish.

1 F. Gary Cunningham, Paul C. MacDonald, Norman F. Grant, et al., Williams Obstetrics, 20th ed. (Stamford: Appleton and Lange, 1997), 30.

heartbeatClick here to try the heartbeat calculator!
Man taking his pulse
Don’t try counting, but your heart will beat more than 3.2 billion (3.2 × 109) times over your lifetime.

Between fertilization and birth, the heart beats approximately 54 million times, while over the course of an 80-year lifespan, the heart beats over 3.2 billion (3.2 × 109) times!3

Play Movie - The Heart in Action
Movie 4.1 - The Heart in Action

Open PDF version of FIG 4.3, Dorsal Aspect of 10 Somite Embryo
Figure 4.3 - Dorsal Aspect of 10 Somite Embryo [PDF version of FIG 4.3]
From Gasser RF, 1975, 24. Atlas of Human Embryos. Copyright © 1975 RF Gasser, PhD.
All rights reserved.


Between 3 and 4 weeks, the body plan emerges as rapid growth of the relatively flat embryo - particularly that of the central nervous system, causes a dramatic folding of the embryo.4 This folding forms the chest and abdominal cavities and incorporates a portion of the yolk sac which becomes the lining of the digestive and respiratory tracts.5 Thus, development of the digestive system is underway just 4 weeks after fertilization.6


Upper and lower limb buds appear by four weeks.7 These buds are the first visible indication of the developing arms and legs. Specialized somites in the embryo’s back begin forming the dermis of the skin and the skeletal muscles, which are used for voluntary movement. These somites will also form the bones of the spinal column, skull, sternum, and ribs.8 Skeletal muscles eventually enable you to move your body.

Play Movie - Folding of the Embryo
Movie 4.2 - Folding of the Embryo
Play Movie - The Limb Buds
Movie 4.3 - The Limb Buds

A transparent sac called the amnion (am-ne-on) which was formed by 2 weeks, begins to surround the entire embryo by four weeks.9 Filled with amniotic (am-ne-ot’ik) fluid, this sterile liquid suspends and protects the developing embryo. This fluid also facilitates lung development, allows swallowing, and later provides nutrients.10

Play Movie - Amniotic Fluid
Movie 4.4 - Amniotic Fluid
Baby and mother swimming in pool.
The buoyancy you feel in water resembles the suspension experienced in the nine months spent in the fluid-filled womb before birth.
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1 Campbell, 2004. 14; Carlson, 2004. 430; de Vries and Saunders, 1962. 96; Gardner and O'Rahilly, 1976. 583; Gilbert-Barness and Debich-Spicer, 1997. 650; Gittenger-de Groot et al., 2000. 17; Kurjak and Kos, 1994. 439; Moore and Persaud, 1998. 77; Navaratnam, 1991. 147-148; O'Rahilly and Müller, 1987. 99; Pansky, 1982. 296; van Heeswijk et al., 1990. 151; Wisser and Dirschedl, 1994. 108.
2 van Heeswijk et al., 1990. 153.
3See Appendix A.
4 Moore and Persaud, 2003. 78.
5 Gasser, 1975. 26; Moore and Persaud, 2003. 78.
6 DiFiore and Wilson, 1994. 221; Fowler et al., 1988. 793; Grand et al., 1976. 793-794, 796, 798; O'Rahilly, 1978. 125; O'Rahilly and Boyden, 1973. 238-239; O'Rahilly and Müller, 1984. 421; O'Rahilly and Tucker, 1973. 6, 8, 23; Streeter, 1942. 232, 235.
7 Gasser, 1975. 49, 59; Moore et al., 2000. 23; O'Rahilly and Gardner, 1975. 11; O'Rahilly and Müller, 1985. 148, 151; O'Rahilly and Müller, 1987. 143; Streeter, 1945. 30; Uhthoff, 1990. 7, 141.
8 Moore et al., 2000. 17.
9 Gasser, 1975. 30; O'Rahilly and Müller, 2001. 80.
10 O'Rahilly and Müller, 2001. 81.