1. Embryo and Fetal Development
Reproductive System
Embryo and Fetal Development

2. Week 1 First Trimester 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.

3. 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.

4. 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.

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

6. First Trimester
Week 5
Embryo looks like a tabpole and 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.

7. First Trimester
Week 6
Embryo is 1/8 of an inch long (the size of a lentil).
Neural tube is closed and brain is beginning to develop.
Heart beats with a regular rhythm.
Basic facial features will begin to appear (nose, mouth, ears).
Digestive and respiratory system begins to form.
Small buds will grow in to arms and legs.

8. 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.

9. Week 8 First Trimester Over ½ of an inch long (size of a kidney bean).
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.
Begins to move, but the mother can’t feel it yet.

10. First Trimester
Week 9
1 in. long (size of a grape) and weighs a bit less than 1/8 of an ounce.
Embryonic tail has disappeared. It is not a tadpole!!!
Nipples and hair follicles begin to form.
Pancreas, bile ducts, gall bladder, and anus are in place.
Internal reproductive organs start to develop.

11. Week 10 First Trimester Vital organs have solid foundation.
Fully separated fingers and toes.
Skeleton and nails begin to form.
Brain will produce almost 250,000 new neurons every minute!!!
If a boy, his testes will start producing testosterone.
Translucent skin
Outer ears are starting to assume their final form

12. Week 11 First Trimester 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.

13. First Trimester
Week 12
Fetus is 3 inches long (size of a lime) 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
Reflexes kick in.

14. 2nd 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.
Fingerprints forming.
For girls, millions of eggs are forming in the ovaries.

15. Week 14 2nd Trimester Fetus the size of a lemon.
Begins using facial muscles and can suck thumb.
For boys, the prostate gland is developing.
For girls, the ovaries move from the abdomen into the pelvis.
Roof of the mouth will be completely formed.

16. Week 15 2nd Trimester Fetus size of an apple.
Eyebrows and scalp hair may make an appearance.
Bone and marrow are continuing to develop.
Ears have almost reached their final position.
Reacts to light.

17. 2nd Trimester
Week 16
Between 4-5 inches long (size of an avocado) and weighs a bit less than 3 ounces.
Can make a fist.
More developed facial muscles may lead to various expressions.
Frequent bouts of hiccups.

18. Week 17 2nd Trimester Fetus size of a turnip.
Fat stores begin to accumulate under the fetus’s skin this week which will provide energy and help keep baby warm after birth.
Cartilage skeleton being replaced by bone.

19. Week 18 2nd Trimester Fetus size of a bell pepper.
Can hear the mother’s heart beating, stomach rumbling, or blood moving through the umbilical cord.
May be startled by loud noises.
Flexes arms and legs.
Mom may start feeling movements.

20. Week 19 2nd Trimester Fetus size of a small tomato
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.
Senses developing and may pick up mother’s voice in conversations.

21. Week 20 2nd Trimester Halfway point.
6 inches long (size of a banana) and weighs about 9 ounces.
Skin is thickening and developing layers
Has thin eyebrows, hair on the scalp
Well developed limbs.
Can swallow.
Meconium, which will become your baby’s first bowel movement after birth, is made in the fetus’s intestinal tract.
Most ultrasounds will show baby’s sex.

22. Week 21 2nd Trimester Size of a carrot.
Absorbs small amounts of sugar from swallowed amniotic fluid.
Bone marrow starts making blood cells (Liver and spleen doing that before).
Kicks and jabs to the womb felt by mother.

23. Week 22 2nd Trimester Size of small squash.
Taste buds begin to develop.
Testes begin to descend from the abdomen.
Uterus and ovaries are in place, complete with a lifetime supply of eggs.

24. Week 23 2nd Trimester Size of a large mango.
Lungs begin to produce surfactant, the substance that allows the air sacs in the lungs to inflate.
Fat production kicks into high gear.

25. Week 24 2nd Trimester Size of an ear of corn.
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.
Complications are frequent and serious.
Bleeding in the brain and impaired vision.

26. Week 25 3rd Trimester Body continues to build up layers of fat.
Hair is beginning to come in.
Continues to explore inside the uterus.

27. Week 26 3rd Trimester 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 yet.
“Practice” breathing moves amniotic fluid in and out.

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

29. 3rd Trimester
Week 28
15 inches long (size of a large eggplant) and weighs about 2 to 3 lbs.
Eyes are beginning to open and close.
Eyesight developing.
Sleeps for about minutes at a time.

30. 3rd 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.
Head growing to make room for brain.

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

32. Week 31 3rd Trimester Size of a coconut.
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.

33. Week 32 3rd Trimester 15-17 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.
Gaining weight quickly in next weeks in preparation for life outside the womb.

34. Week 33 3rd Trimester Size of a pineapple.
Pupils can constrict and dilate in response to light.
Lungs continue to develop.
Babies born this week need extra attention, but almost all will be healthy.

35. Week 34 3rd Trimester Size of a cantaloupe.
Pasty white coating, the vernix, gets thicker.
Central nervous system and lungs maturing.
Most babies do well if born now.

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

37. Week 36 3rd Trimester Size of a head of romaine lettuce.
Gaining about an ounce/day.
Soft, downy hair that covered the fetus under the vernix is now completely gone.

38. 3rd Trimester
Week 37
By the end of this week, the fetus is considered full-term.
Fetus’s body slowly becomes rounder.
Brain and lungs continue to mature.

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

40. Week 39 3rd Trimester Size of mini watermelon
Placenta continues to supply fetus with antibodies that will help the baby fight infection the first six months after.
Physical development complete.
Continue to put on fat to help regulate temperature.

41. 3rd Trimester
Week 40
19-21 in long and weigh 7-8 lbs. (Size of small pumpkin)
It is normal to deliver a baby a week or two late or early.