Stages of Fetal Development

pre-embryonic

• “zygote”

• first 2 weeks after conception

• initiation of cell division (zygote divides while in fallopian tube)

• entry of zygote into uterus

6-10th days

• implantation in decidua

• nidiation: complete implantation (by day 10), gradual process

• implanted usually near time of next expected period = small bleeding

11-14th days

• maintaining decidua

• critically dependent on supply of estrogen & progesterone during secretory phase

• zygote secretes human gonadotropin (HCG) to signal beginning of pregnancy

  • corpus luteum continues to secrete estrogen & progesterone rather than regressing

location of implantation

• site of implantation is the site where placenta develops

• usually occurs in upper uterus (fundus) — best!

  • rich blood supply for optimal fetal gas exchange & nutrition

  • thick uterine lining in upper uterus, preventing placenta from deeply attaching (placenta accreta, percreta, increta)

  • area limits blood loss after birth bc strong interlacing muscle fibers which compress open vessels after placenta separates

embryonic

• “embryo”

• 3-8 wks after conception

• basic structures of all major body organs are all completed during this period

• organs vulnerable to structural damage from teratogens

  • women may not realize the she’s pregnant = birth defects

differentiation of cells

• embryonic period

• progresses from identical functions to differentiated/specialized body cells

• by end of 8th wk, ALL major organ systems in place

development of specialized structures controlled by 

1. genetic info in chromosomes

2. interaction btwn adjacent tissues

3. timing

cephalocaudal

head to toe

central to peripheral

inner to outer

simple to complex

early cells (stem cells) are undifferentiated & have the ability to become any cell in the body prior to becoming specialized

general to specific

embryonic pattern of prenatal growth & development

week 2

• implantation complete

• most growth occurs in outer cells (trophoblast)

• eventually becomes fetal part of placenta

week 3

• many miss their first menstrual period by this time

• CNS begins developing by end of 3rd wk

  • neural tube fused in middle but open at each end

• early heart development begins beating at 21-22 days

week 4

• neural tube completes closure during this week

  • if not, anencephaly & spina bifida occur

• formation of face, upper respiratory tract begins, internal ear & eye

• upper extremities appear as buds on lateral walls

• partition of heart chambers begin at this time & ends by 6th week

week 5

• brain grows rapidly during this time

• heart is beating & continues to develop its 4 chambers

week 6

• heart reaches final 4 chambered form

• eye continues to develop & beginning of external ear, facial features coming together

week 7

• growth & refinement of all systems

• face is now human-like

• intestines grow faster than abdominal cavity

  • intestines contained in umbilical cord

week 8

• external genitalia begin to differentiate

• male & female characteristics are NOT distinct until after 10th wk

fetal

• “fetus”

• 9 wks - birth

• teratogens may damage already formed structures, but are less likely to cause major structural alterations

• CNS - vulnerable to damaging agents throughout entire pregnancy

• rapid growth of fetus & tissue/organ differentiation

• sex distinguishable by 12th wk

• MAY be viable at 22 wks

  • survival not good until several wks older

legal age of viability

20 wks

week 9-12

• first fetal movements occur

  • too slight for mom to feel

• eyes close at 9th wk & open at 26th wk

• blood formation begins in liver & shifts to spleen by end of 12th wk

• begin producing urine

  • excrete into amniotic fluid

• fetal sex can be determined by appearance of external genitalia

week 13-16

significant bone ossification, slow eye movements

week 17-20

• fetal movement feel like fluttering or butterflies

  • women may not recognize

  • quickening at 16-20wks, 18-20wks for primis

• vernix cerosa

• lanugo

• brown fat

vernix cerosa

fatty, cheese-like secretions of fetal sebaceous glands

• covers skin to protect from constant exposure to amniotic fluid

lanugo

fine, downy hair covers fetal body to help vernix adhere to skin

• both diminish as fetus approaches term

brown fat

heat-producing fat deposited on the back of neck, behind sternum, around kidneys

• helps neonate maintain temp stability after birth

week 21-24

• little subcutaneous fat

• skin translucent

• lungs begin to produce surfactant

• by 24 wks, surfactant secreted in alveoli

• fetus more likely to survive if born after 24 wks

surfactant

surface active lipid that makes breathing easier after birth

week 25-28

• eyes: closed at 9 wks, now OPEN

• head: hair abundant

• blood formation shifts from SPLEEN to BONE MARROW

• usually assumes VERTEX (head down) POSITION

week 29-32

• skin pigmented according to race & is smooth

• toenails present

• fingernails extend to fingertips

• if born at this stage, survival is GOOD

week 33-38

• fetal growth slows as fetus approaches term but gains weight

• testes in scrotum

• breasts in both M/F

  • enlarged breast tissue palpable beneath areola & nipple

• firm grasp

• relatively mature nervous system

• maximizes weight gain