Week 4

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My Body in the 4th Week

Trimester 1 of your pregnancy

The missed period that you were expecting at the end of this week will probably be the first major clue that you are pregnant. However, you may notice a little "spotting" or light bleeding when the embryo implants in your uterine lining this week. Tingling, tender, or swollen breasts may be another early sign that you are pregnant.

My Baby in the 4th Week

Baby's cells begin to multiply, and placental tissue grows. At this point, though, the cluster of cells is microscopically small—measuring at most 1 mm in length—and can't yet be seen even on an ultrasound. Nevertheless, 3 different germ cell layers are developing. The ectoderm will produce your baby's nervous system, skin, and hair.

The endoderm will become the epithelial lining of the gastrointestinal tract, liver, pancreas, and thyroid. And the mesoderm will develop into the skeleton, connective tissues, blood system, urogenital system, and muscles.