Medical Definition of Trimester

  • Medical Author:
    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

Trimester: In obstetrics, one of the three divisions of three months each during pregnancy, in which different phases of fetal development take place. The first trimester is a time of basic cell differentiation. The second trimester is a period of rapid growth and maturation of body systems. A second-trimester fetus that is born prematurely may be viable, given the best hospital care possible. The third trimester marks the final stage of fetal growth, in which systems are completed, fat accumulates under the soon-to-be-born baby's skin, and the fetus at last moves into position for birth. This trimester ends with birth.

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Reviewed on 12/27/2018

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