During the first 2-3 weeks of conception, the embryo develops three primary germ layers - ectoderm, mesoderm, and endoderm - which give rise to the skin, muscles, and digestive system respectively. The heart begins beating by the end of the third week. By 4 weeks, the neural tube has closed to form the brain and spinal cord, and the heart begins regularly circulating blood through the main vessels. By 5 weeks, the brain has differentiated into 5 areas and 10 pairs of cranial nerves are present. Maternal changes during this time include breast tenderness, frequent urination, nausea and vomiting, and fatigue.

1. Conception to 8 Weeks NU304B Spring 2009 Meagan Bamba Ada, Mary Joy Aparicio, Kat Arcala, Mary Ann Bartonico, Christine Bernardo

20. Human Chorionic Gonadotropin Released in response to fertilization Detected 2 weeks after conception Prevents mother's immune system from attacking embryo Prevents menstruation Maintains level of progesterone Marker for pregnancy tests

21. 2 nd - 3 rd Week: Primitive Streak A primitive streak is seen during process of differentiation when cells migrate across the surface of the embryo to form the 3 primary germ layers.

22. 2 nd - 3 rd Week: ECTODERM Develops into the skin, hair, nails, nose, ears, mouth, teeth, anal canal, sensory organs, the lenses of the eyes, the retina, brain, and the nervous system.

23. 2 nd - 3 rd Week: MESODERM Develops into the muscles, bones, cartilage, lining of the blood vessels, kidney, heart, lungs, reproductive, circulatory and excretory systems

24. 2 nd - 3 rd Week: ENDODERM Develops into the inner lining of the digestive tract, surface lining of the respiratory passages, trachea, tongue, urethra, bladder and pancreas

25. 2 nd - 3 rd Week: Organ Systems Length : 2 mm from crown to rump Width : Head end is wider than tail end, with a slightly narrowed middle GI System : Liver begins to function Respiratory System : Nasal pits forming Endocrine System : Thyroid tissue appears Eyes : The optic cup and lens pit have formed, pigment in eyes Ears : Auditory pit is now an enclosed structure Cardiovascular : Tubular heart begins to form, blood begins to circulate and by the end of the third week, the heart begins to beat and can be detected on ultrasound!

26. Fetal Development 3 weeks: The embryonic disc becomes elongated and pear-shaped to form a long cylindrical tube called nortochord for brain and spinal cord development. Basically, the beginning of a body. Heart and kidneys begins to form. Liver begins to function. 4 weeks: Anterior portion of neural tube closes to form brain and posterior portion forms spinal cord. Tubular heart begins beating at a regular rhythm and starts pushing its own primitive blood cells through the main blood vessels by the end of 28 days. Oral cavity and stomach begins to form. 5 weeks: Brain has differentiated into 5 areas and 10 pairs of cranial nerves are present Division of the atrium occurs Maternal Changes Breast feels full; tenderness and tingling feeling occurs in the nipples Urinate more often Nausea and vomiting occurs; gum tissue becomes hyperemic and softens and may bleed when only mildly traumatized May be tired, due to hormonal changes and/or anemia (lack of iron)‏

27. 6 Weeks

28. 7 Weeks

29. 8 Weeks