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Maternal Changes During Pregnancy

Noninvasive prenatal tests may also be performed, but they are currently not as informative as amniocentesis and chorionic villus sampling.
The maternal alpha-fetoprotein (AFP) test requires a blood sample from the mother. It is used to detect AFP (a protein produced by the fetus at its highest levels between weeks 12-15) after the 16th week of pregnancy when levels go to zero. High levels at this point indicate a neural tube defect.

During the first 3 to 4 months of pregnancy, the corpus luteum secretes progesterone and estrogens in low levels.
From the 3rd month to the end of the pregnancy, the placenta produces high levels of these hormones.
The chorion secretes human chorionic gonadotropin (hCG) to stimulate the corpus luteum to produce estrogens and progesterone to inhibit menstruation until the placenta takes over.
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