Reproductive System Embryo and Fetal Development

Week 1 You are not actually pregnant!!! Conception typically occurs about two weeks after the menstrual cycle begins. Doctors calculate the due date from the start of the last period. This means the period is counted as part of the pregnancy. First Trimester

Week 2 Sperm and egg unite in one of the Fallopian tubes to form a zygote. Zygote, if normal, has 46 chromosomes. Zygote travels down the Fallopian tube toward the uterus. The inner group of cells will become the embryo. The outer group of cells will become the membranes that nourish and protect it. First Trimester

Week 3 Zygote has now about 500 cells and it is called a blastocyst. The blastocyst will burrow into the uterine wall for nourishment. The placenta, which will nourish your baby, also begins to form. Warning!!! Positive pregnancy test. First Trimester

Week 4 Marks the beginning of the embryonic period. 1/25 of an inch long. Brain, spinal cord, heart, and other organs begin to form. First Trimester

Week 5 Embryo is 1/17 of an inch long, about the size of the tip of a pen. Embryo’s heart and circulatory system are taking shape. Blood vessels form and heart begins to beat. Blood circulation begins, making the circulatory system the first functioning system. First Trimester

Week 6 Embryo is 1/8 of an inch long. Neural tube is closed Heart beats with a regular rhythm. Basic facial features will begin to appear. Digestive and respiratory system begins to form. Small buds will grow in to arms and legs. First Trimester

Week 7 Embryo is a little bigger than the top of a pencil eraser. Weighs less than an aspirin tablet. Umbilical cord is now clearly visible. Arm buds resemble a tiny paddle. Skull is still transparent. Face takes more definition as a mouth, nostrils, and ears indentations become visible. First Trimester

Week 8 Over ½ of an inch long. Webbed finger and toes. Wrists, elbows, and ankles are clearly visible. Eyelids beginning to form. Ears, upper lip and tip of the nose become recognizable. Heart pumps at 150 beats a minute. First Trimester

Week 9 1 in. long and weighs a bit less than 1/8 of an ounce. Embryonic tail is shrinking. It is not a tadpole!!! Nipples and hair follicles begin to form. Pancreas, bile ducts, gall bladder, and anus are in place. Pancreas, bile ducts, gall bladder, and anus are in place. Internal reproductive organs start to develop. Begins to move, but the mother can’t feel it yet. First Trimester

Week 10 Vital organs have solid foundation. Embryonic tail is gone. Fully separated fingers and toes. Skeleton begin to form. Brain will produce almost 250,000 new neurons every minute!!! If a boy, his testes will start producing testosterone. First Trimester

Week 11 Embryo is officially described as a fetus. Blood vessels in the placenta are growing larger and multiplying. Ears are moving up and to the side of the head. Fetus external genitalia will develop into a recognizable penis or clitoris and labia majora. First Trimester

Week 12 Fetus is 3 inches long and weighs about 4/5 of an ounce. Fetus head is nearly half the size of his or her entire body. Fingernails and toenail appear. Chin and nose become more refined First Trimester

Week 13 Fetus moves in a jerky fashion, a mother can’t feel it yet. Fetus might be able to put a thumb in his or her mouth. Eyelids fused together to protect developing eyes. Tissue that will become bone is developing around the fetus’s head and within the arms and legs. 2 nd Trimester

Week 14 For boys, the prostate gland is developing. For girls, the ovaries move from the abdomen into the pelvis. Meconium, which will become your baby’s first bowel movement after birth, is made in the fetus’s intestinal tract. Roof of the mouth will be completely formed. 2 nd Trimester

Week 15 Baby skin starts out nearly transparent. Eyebrows and scalp hair may make an appearance. Bone and marrow are continuing to develop. Ears have almost reached their final position. 2 nd Trimester

Week 16 Between 4-5 inches long and weighs a bit less than 3 ounces. Can make a fist. Eyes are becoming sensitive to light. More developed facial muscles may lead to various expressions. Frequent bouts of hiccups. For girls, millions of eggs are forming in the ovaries. 2 nd Trimester

Week 17 Fat stores begin to accumulate under the fetus’s skin this week. Fat will provide energy and help keep baby warm after birth. 2 nd Trimester

Week 18 Can hear the mother’s heart beating, stomach rumbling, or blood moving through the umbilical cord. May be startled by loud noises. Can swallow. 2 nd Trimester

Week 19 Skin is now covered with a pasty white coating called vernix. Udder the vernix, a fine, down-like hair called lanugo covers the fetus. Kidneys produce urine, which is excreted into the amniotic sac. May pick up mother’s voice in conversations. Makes reflexive muscle movements. 2 nd Trimester

Week 20 Halfway point. 6 inches long and weighs about 9 ounces. Mother begins to feel movement. Skin is thickening and developing layers Has thin eyebrows, hair on the scalp Well developed limbs. 2 nd Trimester

Week 21 Absorbs small amounts of sugar from swallowed amniotic fluid. Bone marrow starts making blood cells. Who did it before? –Liver and spleen. 2 nd Trimester

Week 22 Taste buds begin to develop. Brain and nerve endings can process the sensation of touch. Testes begin to descend from the abdomen. Uterus and ovaries are in place, complete with a lifetime supply of eggs. 2 nd Trimester

Week 23 Lungs begin to produce surfactant, the substance that allows the air sacs in the lungs to inflate. “Practice” breathing moves amniotic fluid in and out. Fat production kicks into high gear. Begins to look more like a newborn baby. 2 nd Trimester

Week 24 Footprints and fingerprints are forming. Fully developed inner ear allows fetus to have a sense of position. Regular sleeping and waking cycle. If born has more than a 50% chance of survival. Complication are frequent and serious. –Bleeding in the brain and impaired vision. 2 nd Trimester

Week 25 Hands are now fully developed. Continues to explore inside the uterus. 3 rd Trimester

Week 26 Weighs 2 lbs. Eyebrows and eyelashes are well formed. Hair on the head is longer and more plentiful. Eyes are fully developed, but may not open for another two weeks 3 rd Trimester

Week 27 Lungs, liver, and immune system continue to mature. If born this week, the chance of survival is at least 85%. 3 rd Trimester

Week inches long and weighs about 2 to 3 lbs. Eyes are beginning to open and close. Sleeps for about minutes at a time. 3 rd Trimester

Week 29 Bones are fully developed, but they are soft and pliable. Begins to store iron, calcium, and phosphorus. Movement becomes more frequent and vigorous. 3 rd Trimester

Week 30 Weighs about 3 lbs. May practice breathing by moving his or her diaphragm in a repeating rhythm. 3 rd Trimester

Week 31 Testicles begin moving from near the kidneys into the scrotum. Clitoris is now relatively prominent. Lungs are more developed. If born, baby will need a ventilator to assist breathing. 3 rd Trimester

Week inches long and weighs about 4 lbs. Layer of soft, downy hair starts to fall off. Nearly all babies born this week survive. Running out space. If you count fewer than 10 movements in two hours, contact health provider. 3 rd Trimester

Week 33 Pupils can constrict, dilate and detect light. Lungs continue to develop. Babies born this week need extra attention, but almost all will be healthy. 3 rd Trimester

Week 34 Pasty white coating, the vernix, gets thicker. Soft, downy hair that covered the fetus under the vernix is now completely gone. 3 rd Trimester

Week 35 Fetus continues to pack on the pounds and store fats. Lots of stretches, rolls and wiggles. 3 rd Trimester

Week 37 By the end of this week, the fetus is considered full- term. Fetus’s body slowly becomes rounder. 3 rd Trimester

Week 38 Weighs nearly 7 lbs. CNS are working better every day. 3 rd Trimester

Week 39 Placenta continues to supply fetus with antibodies that will help the baby fight infection the first six months after 3 rd Trimester

Week in long and weigh 7-8 lbs. It is normal to deliver a baby a week or two late or early. 3 rd Trimester