Blastocyst Development• Zygote (fertilized egg) marks beginning of development• Zygote develops into blastocyst

- Ball of cells- ~ 5 days after fertilization

• Hormones (progesterone) released by corpus luteum help zygote to develop and keep the uterine lining thick

Gastrula Development• Blastocyst implants into

endometrium (lining of uterus)

• Continues development Gastrula

– Ectoderm becomes skin & nervous system

– Mesoderm becomes muscles, blood, & internal tissues

– Endoderm becomes digestive & internal organs

Embryo DevelopmentBlastocyst Gastrula Embryo • Embryonic membranes protect & nourish embryo

• Endometrium becomes placenta- Connects mother and embryo

• Umbilical cord develops- Connects embryo to placenta- Blood vessels from placenta to baby

• Mom’s & baby’s blood do not circulate together!

• Can have 2 different blood types• Nutrients move by diffusion

• Amniotic sac cushions embryo- Amnion = membrane surrounding baby- Amniotic fluid = “water” surrounding baby

• “water breaks” before baby is born (membrane ruptures and fluid escapes)

Fetal DevelopmentZygote Blastocyst Gastrula Embryo Fetus• Fully formed fetus takes 38-40 weeks to develop• Human pregnancies are divided into trimesters

Fetal Development• First trimester (weeks 1-12):

- Body plan & early development• Heartbeat detected after

week 5• Arms and legs begin to

develop• Heart, brain, pancreas,

kidneys, liver begin to form• Eyelids form then fuse for

iris to develop• Hair, fingernails, and

toenails develop• External sex organs show

sex of fetus (at end of trimester)

Fetal Development• Second trimester (weeks 13-27):

- Fetus more active & developed• Most joints and bones have

started to form• Skin is protected by fine air

and waxy substance• Sleep cycles more regular• Eyes open & blink• Eyebrows/eyelashes form• Fetus breathes amniotic

fluid (strengthens lungs)• First movements felt by

mother

Fetal Development• Third trimester (weeks 28-40):

- All organs fully form• Fetus responds to light &

sound (from outside uterus)• Fetus has periods of

dreaming• Fine body hair thins, scalp

hair grows in• Bones grow & harden• Synapses between neurons

form in huge numbers• Lungs complete

development• Fetus turns head-down

(ready to exit)

Birth• Baby is upside down

– Feet first = breech position (not good)• Cesarian-section (“C-

section”)• Labor

– Muscles help to thin & open cervix

– Muscle contractions push baby out

• After birth- placenta detaches from uterus and exits

https://www.youtube.com/watch?v=-jlPdMKPyZY

The mother affects the fetus, and pregnancy affects the mother.

• The fetus depends on the mother for all its nutrition.– mother's diet must support fetal health– mother must avoid raw foods & toxic chemicals

• Pregnancy affects mother– increase in Calories and body

weight – pregnancy hormones affects

homeostasis – regular medical checkups

needed

Review1. Define: blastocyst, gastrula, embryo2. Explain the functions of the following:

PlacentaUmbilical cordAmniotic sac

3. List at least 2 milestones for each trimester of pregnancy.4. What might happen if the baby’s head is not upside down when ready for

birth?5. Give 2 examples of how a mother and fetus affect one another.6. Why might a pregnant woman need to be concerned about what she

eats and/or drinks?7. A baby is born 12 weeks premature (early). The organs are developed,

but the baby must breathe using a ventilator. Explain why this might be necessary.