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Gestational age: 4 weeks and 0 days until 4 weeks and 6 days old. 29–35 days from last menstruation. Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization. A notochord forms in the center of the embryonic disk. (day 16 of fertilization. [1]) Gastrulation commences. (day 16 of fertilization. [1])
The first ten weeks of gestational age is the period of embryogenesis and together with the first three weeks of prenatal development make up the first trimester of pregnancy. From the 10th week of gestation (8th week of development), the developing embryo is called a fetus.
Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation (pregnancy) is about nine months or 36 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus.
About 80% of miscarriages occur in the first 12 weeks of pregnancy. The underlying cause in about half of cases involves chromosomal abnormalities. [157] Stillbirth is defined as fetal death after 20 or 28 weeks of pregnancy, depending on the source. It results in a baby born without signs of life.
Mammals during pregnancy can have one or more gestations at the same time, for example in a multiple birth. [2] The time interval of a gestation is called the gestation period. In obstetrics, gestational age refers to the time since the onset of the last menses, which on average is fertilization age plus two weeks. [3]
The two pre-implantation stages are associated with the pre-implantation embryo. [9] [10] In humans, following the stage of hatching that takes place around four to five days after fertilization, the process of implantation begins. By the end of the first week, the blastocyst is superficially attached to the uterine endometrium. By the end of ...
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