MCAT Biology Review

Chapter 3: Embryogenesis and Development

3.4 Gestation and Birth

Human pregnancy lasts an estimated 266 days, which are divided into three trimesters. As a general rule, the larger the animal, the longer the gestational period and the fewer the offspring per pregnancy. For example, elephants usually have one calf and gestate for 22 months. In contrast, mice have 10 to 12 offspring per litter and gestate for only 20 days. Although you don’t need to know every detail of gestation for the MCAT, there are some key developmental events in each trimester with which you should be familiar.


During the first weeks, the major organs begin to develop. The heart begins to beat at approximately 22 days, and soon afterward, the eyes, gonads, limbs, and liver start to form. By five weeks, the embryo is 10 mm in length, and by week six, it has grown to 15 mm. The cartilaginous skeleton begins to harden into bone by the seventh week. By the end of eight weeks, most of the organs have formed, the brain is fairly developed, and the embryo becomes known as a fetus. At the end of the third month, the fetus is about 9 cm long.


During the second trimester, the fetus undergoes a tremendous amount of growth. It begins to move within the amniotic fluid, its face takes on a human appearance, and its toes and fingers elongate. By the end of the sixth month, the fetus measures 30 to 36 cm long.


The seventh and eighth months are characterized by continued rapid growth and further brain development. During the ninth month, antibodies are transported by highly selective active transport from the mother to the fetus for protection against foreign matter in preparation for life outside the womb. The growth rate slows and the fetus becomes less active, as the it has less room to move about.


Vaginal childbirth is accomplished by rhythmic contractions of uterine smooth muscle, coordinated by prostaglandins and the peptide hormone oxytocin. Birth consists of three basic phases. First, the cervix thins out and the amniotic sac ruptures, which is commonly called water breaking. Next, strong uterine contractions result in the birth of the fetus. Finally, the placenta and umbilical cord are expelled; these are often referred to as the afterbirth.


Advances in medicine have allowed premature babies born as early as 24 weeks to survive—far short of the normal 40 weeks. While these neonates may survive, there are often severe complications because fetal development is not complete at 24 weeks. These problems are most apparent in the respiratory, gastrointestinal, and nervous systems.

MCAT Concept Check 3.4:

Before you move on, assess your understanding of the material with these questions.

1.    What are some of the key developmental features of each trimester?

·        First trimester:

·        Second trimester:

·        Third trimester:

2.    What occurs in each of the three phases of birth?