Conception & Fetal Development

gametogenesis

• 46 chromosomes

• 22 paired autosomes + one sex specific pair of chromosomes (x or y) = 23 pairs

• each parent provides one

• male (father) determines the sex of the baby

ovum

• females have all their eggs at birth(millions)

• 400-500 will mature and release (puberty to menopause)

• only viable for 24hrs

sperm

• produced in testes at puberty (not present at birth)

• produced throughout life

• viable for 2-3 days

ovum transport

• fallopian tube contracts before & during ovulation

• fimbriae cover site of ovulation

• captures ovum

• cilia in fallopian tube move egg into tube

• tube transports ovum (if fertilized then zygote) to uterus

sperm transport

• deposit in vagina near cervix

• tail of sperm whips & moves sperm quickly (4-6hrs to site of fertilization, some in 5min)

• sperm are in fallopian tubes w/i minutes of insemination

fertilization

• occurs in distal 1/3-2/3 of fallopian tube (ampulla)

• 1 sperm per egg

• nuclei fuse to form zygote

• 3-4 days it travels through tube into uterus

changes in zygote

• rapid cell division

• at 16 cells - called the morula

• days 3-4 - morula floats inside uterine cavity as it continues to divide

• blastocyst forms inside morula (future embryo & amnion)

• trophoblasts is the outer layer of the morula (future chorion & placenta)

• fluid separates these 2 layers

• day 6 is implantation of the zygote

implantation

• occurs 6-7 days after fertilization

• chorionic villi develop from trophoblasts & embed into endometrium for nourishment

• now called decidua basalis becomes maternal side of placenta

• usually occurs in anterior or posterior part of fundus (can occur in other places)

• corpus luteum - area on ovary where egg was released & it produces progesterone

• human chorionic gonadotropin (HCG) is produced by chorionic villi (placenta) for 50-70 days so it can sustain corpus luteum (on ovary) to produce progesterone till placenta is able to

• single cell layer separates fetal & maternal blood

embryonic membrane

• chorion - develops from trophoblasts; contains chorionic villi (disappear everywhere except placenta); contains embryo, yolk sac, & amnion

• amnion - surrounds embryo & is w/i chorion; eventually contains 500-1000 mL of fluid; expands w/fetal growth eventually touching chorion

• embryonic stage - 15 days - 8 wks; organogenesis completes in this time; teratogens detrimental to development of organs

• fetal stage - 9 wks - delivery

functions of amniotic fluid (AF)

• maintain body temp

• oral fluid & repository of wastes

• F&E maintenance

• cushion

• barrier to infection from vagina

• allows fetal lung development

• can be w/drawn to test for genetic abnls, fetal lung maturity, presence of infection, etc.

• oligohydramnios - less than 300 mL of AF

• hydramnios (polyhydramnios) - 2 L or more of AF

placenta

• maternal side develops from decidua basalis & fetal side from chorion

• 20 cm diameter

• 2-3 cm thick

• ~1 lb at birth

• fetal surface smooth, shiny, gray

• maternal surface red, fleshy

• 15-20 segments called cotyledons

• hormonal function (produces hCG, hPL, progesterone, estrogen)

• metabolic function (transfers gases, nutrients, heat, excretes fetal waste, transports passive immunity

progesterone

• secreted by corpus luteum then placenta

• greatest amounts during second phase of menstrual cycle (days 15-26)

• decreases contractility of uterus/fallopian tubes (keeps relaxed)

• causes endometrium to increase in blood supply, glycogen, amino acids, & water

• increases thickness of cervical secretions

• increases temperature

what decreases placental blood flow

• decreased maternal BP

• compression of VC

• vasoconstriction

• excessive exercise

• prolonged contraction length too close together

• contractions

umbilical cord

• 40-70 cm long

• should contain 1 vein & 2 arteries

  • vein carries oxygenated blood from mom to fetus

  • arteries carry deoxygenated blood from fetus to mom’s circulation

• vessels surrounded by Wharton’s jelly (firm & gelatinous)

• Nuchal cord - cord wrapped around fetal neck

fetal development

• cephalocaudal (head to toe)

fetal circulation

• fetal lungs don’t function for respiratory gas exchange, so a special circulatory pathway, ductus arteriosus, bypasses the lungs. Oxygen-rich blood from the placenta flows rapidly through the umbilical vein into fetal abdomen. When umbilical vein reaches liver, it divides into 2 branches; one branch circulates some oxygenated blood through liver. Most of the blood passes through the ductus venosus into the IVC. There it mixes w/deoxygenated blood from fetal legs/abdomen on its way to right atrium. Most of this blood passes through RA & through foramen ovale, an opening into LA. There it mixes w/deoxygenated blood returning from fetal lungs through pulmonary veins.

• all (ductus venosus/arteriosus, umbilical vein/artery, & foramen ovale) should disappear soon after birth

fetal maturation-hematopoietic system (“the blood system”)

• 3rd wk - hematopoiesis begins in yolk sac

• 5th wk - hematopoietic stem cells “seed” fetal liver

• 6th wk - blood cell production occurs in fetal liver

• 8-12 wks - bone marrow, spleen, & thymus are “seeded”

• 6th wk - antigenic factors present to determine fetal blood type

• 9th-10th wk - glycogen is stored in liver, major source of energy

fetal maturation - respiratory

• 32 wks = surfactant (helps lungs not collapse)

• synthetic surfactant can be instilled in lungs at birth if needed

fetal maturation - GI

• begins swallowing amniotic fluid at 5 wks causing peristalsis to begin

• produces bile at 12 wks

• meconium - first stool (contains bile)

• 26 wks begins laying down brown fat

• GI mature by 36 wks

fetal maturation - hepatic

• glycogen store in liver by 9-10 wks

• iron - store in liver, can store 5 months worth

• inadequate amount of enzyme to conjugate bilirubin

• cannot synthesize clotting factors

fetal maturation - renal

• kidneys form in 15th wk

• difficulty concentrating urine at birth

fetal maturation - neurologic

• brain waves can be recorded by wk 8

• at birth brain is ¼ size of adult

fetal maturation - sensory

• purposeful movements indicate ability to detect pain/pressure

• responds to sound by 24 wks

• see by 7 months & have REM sleep

• can respond to taste/temp changes in utero

fetal maturation - endocrine

• thyroid gland developed by 34th wk & secretion of thyroxin by 8th wk

• adrenal cortex - 6wk & produces hormones by 8 wks

• pancreas - 5-8 wk. Islet of Langerhans (insulin producer by wk 20)

fetal maturation - reproductive

• sex differentiation by wk 9-12

• by wk 16 oogenesis is established (eggs)

• hormones produces can cause

  • pseudomenstruation in females

  • mammary engorgement & secretion of “which’s milk” in both sexes

fetal maturation - integumentary

• epidermis - 4 wks

• vernix caesosa - sloughed epidermal cells + sebaceous gland secretions from this white thick coating on fetus at ~24 wks

• 16 ridges on palms/feet

• lanugo - fine hair appears ~12 wks

• nails grow to tips by 32-36 wks

organ system development

• 4 wks - double-chambered heart first starts beating

• 8 wks - inner ear development

• 12 wks - sex can be determined

• 12 wks - kidneys can secrete urine

• 12 wks - sucking present

• 24 wks - baby can hear

• 24 wks - eyelids open

• 30 wks - testicles descend in scrotum

fetal maturation - immunologic

• IGG - only immunoglobulin to cross placenta giving passive immunity

• IGM - produced by fetus by wk 13

• IGA - not produced by fetus, however colostrum has large amounts of IGA passing along passive immunity if baby breastfeeds

length of pregnancy

• lasts ~ 10 lunar months

• lunar months have 4 wks

• pregnancy should be ~ 40 wks or 280 days

embryo & fetal development

• 3 stages of development:

  • pre-embryonic - first 2 wks

  • embryonic: from day 15 - 8th wk

  • fetal stage - from end of 8th wk - delivery

• viability - age at which fetus can survive outside the uterus (usually ~20 wks & 350-500g in weight)

teratogens

• malformations during the period or organogenesis (days 15-60) cused by external factors & substances

• examples - infections, drugs, environmental influences, & maternal conditions (diabetes or PKU)

twins - dizygotic

• fraternal twins

• not true twins

• maturation of 2 eggs during ovulation

• can be different sex

• 2 separate chorions, amnions, & placentas

twins - monozygotic

• true twins - “identical”

• division of one zygote into 2

• share same chromosomes but not exact amounts of protoplasm

• most have own amniotic sac but usually share one chorionic sac & one placenta