Stages of Development

Overview Development of the embryo is broken into 3 Trimesters each with 12 weeks Up to the 9th week=embryo Beyond 9th week=fetus At 36-37 weeks, safe to deliver 40 weeks considered full term Ovarian Cycle Fetal Development Stages – Sutterhealth Visible Embryo – PBS Medline Plus

1 Day Post-Ovulation Sperm fertilizes egg Creation of ZYGOTE

1.5 – 3 Days Post-Ovulation Cleavage of zygote-picture Development Picture Cleavage of zygote-picture First cell divisions repeat approximately every 20 hours When cell division creates about 16 cells, the mass is called a MORULA Morula leaves oviduct to uterus 3-4 days after fertilization

Day 4 (Week 1) Early Blastocyst phase Development Picture Early Blastocyst phase In uterus, cell division continues Cells move to outer areas leaving open cavity inside—this is called the BLASTOCYST (Blastula)

Day 5-6 (Week 1) Implantation Begins Considered pregnant Development Picture Implantation Begins Considered pregnant Progesterone produced to maintain thickness of uterine lining

Day 7-12 (Week 2) Implantation complete Growth of placenta begins Development Picture Implantation complete Growth of placenta begins Inner cells divide rapidly forming a two layered disc Top layer will become embryo and amniotic cavity Bottom layer becomes the yolk sac

(Week 2) Gastrulation and formation of chorionic villi (“fingers” that form placenta anchor to uterus) Formation of blood and blood vessels Gastrulation, a process that gives rise to all three tissue layers of the embryo: ectoderm, mesoderm and endoderm. Gastrulation Picture DAY 16

Tissue Layers Endoderm form the lining of lungs, tongue, tonsils, urethra and associated glands, bladder and digestive tract. Mesoderm form the muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, and reproductive and excretory systems. Ectoderm form the skin, nails, hair, lens of eye, lining of the internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands, and all parts of the nervous system.

Week 3 Beginning development of the brain, spinal cord, and heart Nervous system first to develop Beginning development of the gastrointestinal tract                                  1st Month Diagram

Week 4-5 Beginning of vertebra and bones Development of the heart which now beats at a regular rhythm Movement of basic blood through the main vessels Beginning of the structures of the eye and ears Brain develops into five areas and some cranial nerves are visible Arm and leg buds are visible

Week 6 Beginning of formation of the lungs Further development of the brain Arms and legs have lengthened with foot and hand areas distinguishable Hands and feet have digits, but may still be webbed

Week 7-8 Nipples and hair follicles form Elbows and toes visible All essential organs have at least begun to form Facial features continue to develop Eyelids are more developed External features of the ear begin to take their final shape

Week 9-12 Fetus reaches a length of 3.2 inches Head comprises nearly half of the fetus' size Development of four-chambered heart Fingernails formation begins Tooth buds appear for the baby teeth Fetus can make a fist with its fingers Genitals appear well differentiated Vocal cords can make sounds Bone hardening begins First detectable brain waves-Involuntary movements possible

Week 13-16 Fetus about 6 inches Fine hair develops on the head called lanugo More muscle tissue and bones have developed, and the bones become harder Fetus makes active movements Sucking motions are made with the mouth Muconium is made in the intestinal tract

Week 17-20 Fetus reaches a length of 8 in Lanugo hair covers entire body Eyebrows and lashes appear Nails appear on fingers and toes Fingerprints formed More active with increased muscle development-mom can feel the fetus moving Taste buds formed

Week 21-24 Fetus length of 11.2 inches Weighs about 1 lb. 10 oz. All the eye components are developed Hand and startle reflex Air sacs forming in lungs Thumb sucking Rapid brain growth Can sense light/sound

Week 25-28 Length of 15 inches; about 2 lbs. 11 oz. Nervous system developed enough to control some body functions Eyelids open and close Respiratory system, while immature, has developed to the point where gas exchange is possible Baby born at this time may survive, but the possibilities for complications and death remain high

Week 29-32 Length of about 15-17 inches-about 4 lbs. 6 oz. Rapid increase in the amount of body fat Rhythmic breathing movements occur, but lungs are not fully mature Bones are fully developed, but still soft and pliable Eyes open when active and shut when asleep Fetus begins storing minerals

Week 33-36 Length of about 16-19 inches; 5 lbs. 12 oz. to 6 lbs. 12 oz. Lanugo begins to disappear Increase in body fat Fingernails reach the end of the fingertips Baby born at 36 weeks has a high chance of survival, but may require some medical interventions

Week 37-40 19 to 21 inches in length Lanugo is gone except for on the upper arms and shoulders Fingernails extend beyond fingertips Small breast buds are present on both sexes Head hair is now coarse and thicker Lungs finalize development

Development Comparison

1st Month BACK

Cleavage BACK BACK to Week 2