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Female Reproductive SystemTrue Developmental TimelineFetal sexual organs visibleBasic Brain Structure in PlaceFirst Detectable Brain WavesHindbrain Begins to DevelopEsophagus FormingSomites appearHeartbeat can be detectedEnd of embryonic periodUpper limb buds appearFertilizationThird Trimester - Infant PeriodSecond Semester - Fetal Period
first trimester: from zygote to embryo
maternal symptoms... and possible causes...
Morning sickness • The hormone estrogen increases a hundredfold during pregnancy.
• The hormone progesterone also increases as it is needed to relax the muscles in the uterus to prevent early labor.
However, progesterone also relaxes the stomach and intestines, leading to excess acids not being pushed out of the digestive tract.
• The hormone human chorionic gonadotropin (hCG) appears in response to fertilization.
hCG is later stimulated to increase when syncytiotrophoblast cells (the first cells of the placenta) break down maternal uterine cells and implant the zygote. hCG maintains the level of progesterone, as hCG may prevent the mother's immune system from attacking the embryo.
Frequent urination Frequent urination begins about six weeks into pregnancy and is one of pregnancy's most common early signs. The amount of blood in a pregnant woman's body increases by 50% during pregnancy, mainly in response to hormone changes. Eventually the growing fetus and baby, as well as the mother's body, create extra fluids that are processed through the mother's kidneys, ending up in her bladder.
Sleepiness Hormonal changes can cause mothers to feel tired and sleepy.
Food cravings The extreme hormonal changes of pregnancy can affect taste and smell. Women going through menopause also experience strong food cravings and aversions due to hormonal swings. However, cravings to eat non-food items such as dirt, ashes, clay, chalk, ice, laundry starch, baking soda, soap, toothpaste, paint chips, plaster, wax, hair, coffee grounds, and even cigarette butts, can be a sign of severe iron deficiency. Tell your doctor.
Breasts may swell The hormones estrogen and progesterone cause increased blood flow and breast tissue changes, one of the earliest signs of pregnancy. Usually starting around 4 to 6 weeks after implantation and lasting through the first trimester.
Weight gain or loss The full term baby will weigh about 7 1/2 pounds, the amniotic fluid, placenta, and extra body fluids and blood for mother and baby add up to another 8 to 12 pounds. Also, the enlarging uterus and breasts, may add another 4 to 6 pounds.

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roll over chart for explanation of fetal developmental progress