Fetal development

Dr. Carmen Rexach

Physiology

Mt San Antonio College

We all start as one fertilized oocyte!

Gestation lasts ≈266 days

Trimesters: Clinical Approach

8 weeks 5 months

Full term

2nd trimester = 13-24 weeks

3rd trimester = 25 weeks to birth

1st trimester = 1st 12 weeks

Stages of Development

Pre-embryonic

Zygote 0-30 hr A single diploid cell

Cleavage 30-72 hr Identical blastomeres

Morula 3-4d Ball of 16+ blastomeres

Blastocyst 4-16d Trophoblast + inner mass

Embryonic 16d-8wks Differentiation of 1o germ layers

Fetal 8-40wks Growth and maturation

First trimester

8 weeks

12 weeks

7 weeks

30 days post fertilization (3 weeks)

• Four limb buds are visible

• Lens disc & optic vesicle are formed and you can tell future eye color

• body is covered with first thin skin layer

• brain differentiates into forebrain, midbrain, hindbrain

• thyroid development in progress

• Lymphatic system forms

• hearing development begins

30 days post fertilization

• Primitive mouth and tongue have formed • heart chambers are full of plasma and

blood cells • liver has formed and receives blood from

the umbilicus • blood circulation has established • gall bladder, stomach, intestines,

pancreas and stomach form

7 to 8 weeks • First detectable brain waves

• head begins to become erect

• semicircular canals form

• gonads form – sex is visibly detectable by 8 weeks

• Legs formed with recognizable knees and ankles

• Toes are notched and toe nails begin to appear

• trunk elongates

• muscles continue to develop and grow strong

12 weeks (2nd trimester begins)

• Fetus begins to move around

• Neck and shoulders are well-defined

• salivary glands function and sucking muscles of mouth are present

• tooth buds still developing

• hair pattern can be detected on the scalp

• heartbeat can be detected externally

3 months

12 weeks (2nd trimester begins)

• fetus inhales and exhales amniotic fluid which aids in lung development

• 25 quarts of blood are pumped by heart each day

• Fully functional spleen under the direction of the liver

• Limbs are more proportional – Hands and thumbs show increased function

• sweat glands appear • body hair begins to grow • muscle and nerves continue to advance

Second trimester

4 months

5 months

Second trimester

• 14 weeks – increased flexibility

– eyes move forward in the face and ears are in correct position

– torso grows rapidly

12 weeks

22 weeks – fetus occupies more space in uterus -- becomes

crowded

– fetus recognizes maternal sounds -- mother’s voice, heartbeat, breath, digestion

• auditory ossicles have hardened

– continued development of respiratory system

– continued growth of bones, muscles and organs

– skin is pink, wrinkled, with blood vessels, bones, and organs visible through the transparent surface

3rd trimester

6 months

Third trimester

• 28 weeks – brain waves function like full term baby

– forebrain enlarges

– eyelashes present and eyes partially opened

– improved sucking and swallowing

– lungs can breathe air

– 2-3% body fat

– testes descend in male

40 weeks = Full term

– 15% of body is fat (80% under skin, 20% holding organs in place)

– 300 bones -- some will fuse postpartum to make 206 in adult

– over 70 reflex apparent

Amniocentesis • Diagnostic test used to:

– Chromosomal abnormalities • Down’s syndrome

• Spina bifida

• anencephaly

– Rh abnormalities

– Lung maturity