7.1: Fetal Growth and Development Flashcards
Define the ‘fetal period’ and when it occurs
This occurs from the end of 8th week-term. Growth and physiological maturation of structures created during the embryonic period and prepares for the transition to independent life after birth.
How long are the following stages?
a) pre-embyronic
b) embryonic
c) fetal
a) 1-2 weeks
b) 2-8/9 weeks
c) 9-38 weeks
Describe the development and growth occurring during the embryonic period
It has intense activity; organogenetic period where the three germ layers of the embryo form into the major organs of the body system.
There is small absolute growth other than the placenta
Describe the growth pattern of weight gain. In which stage/period does maximum growth and weight gain occur?
Weight gain is slow at first but increases rapidly in the mid-late fetal periods, initially with the deposition of protein and then later with adipose laid in subcutaneous and abdominal stores
Define CRL, when and why is it measured? What is its growth pattern?
Crown-rump length; measurement of the length of human embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump), measured between 7-13 weeks to date the pregnancy and estimate EDD using a scan
CRL increases rapidly in all three periods
What is most significant of the following stages metabolically and in terms of growth?
a) embryo
b) early fetus
c) late fetus
a) intense morphogenesis and differentiation, little weight gain but significant placental growth
b) protein deposition
c) adipose deposition
List three things that can be used to perform an ante-natal assessment of the fetus and when they might be used
- Ultrasound scan in 1st trimester
- Asking the mother if she’s felt fetal movements (after ~17 weeks onwards)
- Regular measurements of the uterine expansion (symphysis-fundal height); second and third trimester
When are obstetric ultrasound scans routinely carried out and what two major things can they assess for?
Routinely carried out at 12 weeks and 20 weeks to assess
- Fetal growth (can also measure CRL and provide an approx date of delivery)
- Fetal anomalies (including the location of the pregnancy/ectopic, number of fetuses, etc).
Regular scanning is offered afterwards for mother’s with health conditions or previous fetal anomalies
How would you provide an EDD to a patient?
Must provide the one calculated by the scan (even if the woman says it doesn’t line up with her LMP) unless its an IVF
What is the biparietal diameter? What can it be used for and when?
The distance between the parietal bones of the fetal skull, it’s used in combination with other measurements (like femur length and abdominal circumference) to date pregnancies and determine any anomalies in T2 and T3
What is the general classification of birth weights (average, growth restriction and macrosomia)?
3 kg is average
<2.5 kg suggests growth restriction
>4.5 kg is macrosomia
Name 3 reasons a baby might have a low birth weight
- Premature
- Constitutionally small
- They have suffered growth restriction
Why is it important for a baby to be delivered at ‘40+12’ weeks
Placental bloodflow becomes more compromised and there is increased risk of stillbirth afterwards
When does the respiratory system develop in relation to other systems? When is the bronchopulmonary tree formed and when does functional specialization occur?
Development is relatively late
- Embryonic development creates the bronchopulmonary tree
- Functional specialization occurs in the fetal period
What does the lung/respiratory bud arise from and what structure is responsible for separating the trachea and esophagus?
The lung bud arises from the respiratory diverticulum (pouches off the esophagus) and is separated from the esophagus by the tracheoesophageal septum
What is a tracheoesophageal fistula? What is a complication for the fetus that arises from this and how might this be recognized once the baby is born?
A connection between the esophagus and the trachea. This prevents the amniotic fluid from passing normally through the GI tract, and excessive amounts may accumulate in the lungs resulting in polyhydramnios. This can make breathing difficult for the baby once it’s born.
Where does gas exchange occur before the baby is born and why? What ‘bodily preparations’ are made in the fetus to assist the lungs in taking over?
Conducted at the placenta (as the developing lungs are fluid-filled, but lungs must be prepared to assume the full burden at birth). The respiratory musculature is pre-conditioned so the baby undergoes “breathing movements” before it is born
What are the fetal implications when the mother has anhydramnios?
A lack of amniotic fluid means the baby’s lungs are unlikely to develop properly and the baby won’t be able to move around comfortably and may develop contractures
What determines the ‘threshold of viability’ of how early a baby can be delivered? What other factors are taken into consideration?
The limit is determined by whether the lungs are sufficiently developed to sustain life, this is only possible if they have entered the terminal sac stage of development, i.e 23-24 weeks. The baby’s weight (must be >5kg) is also taken into consideration
What causes respiratory distress syndrome and what can be given to prevent this?
Typically when a baby is born prematurely and there is insufficient surfactant production. If pre-term delivery is unavoidable can give dexamethasone injections to increase surfactant production in the fetus
What does the fetal CVS ensure? When is the definitive fetal HR achieved and what is fetal bradycardia associated with?
That oxygenated blood collected by the umbilical vein at the placenta is circulated around the fetus. The definitive HR is achieved around 15 weeks and continual fetal bradycardia is associated with fetal demise
What is the functional fetal kidney called, when does it begin functioning and what happens if it is dysfunctional?
The metanephros begins functioning at week 10, if it’s dysfunctional - oligohydramnios will occur as fetal urine is a major contributor to amniotic fluid volume (although fetal kidney function is not necessary for survival in utero)
What abnormalities is oligohydramnios suggestive of?
- Reduced renal function
- Placental insufficiency; such as in pre-eclampsia, as this results in the blood flow being redistributed to the fetal brain rather than the abdomen and kidneys, resulting in poor urine output.
What two things must be checked if a woman presents with oligohydramnios?
- Ensure fetal growth isn’t restricted (i.e if the cause relates to placental insufficiency)
- Check the membrane hasn’t been ruptured
Why is polyhydramnios associated with diabetic moms?
The abundance of glucose allows the baby to grow and pee more (urine is a major contributor to amniotic fluid volume)
Name 3 potential complications for birth when the mother has polyhydramnios
- Placental rupture from the excessive pressure
- Early labour; as the increased fluid causes the uterus to stretch quicker and more than usual and it may begin contracting early
- The excessive fluid also allows the baby to move more freely in the uterus. However, if the water breaks, this can also mean that the baby will be abnormally positioned. The gushing pressure may also mean the baby’s limbs (still attached to rest of baby) and the chord has the potential to come out of the cervix with the water
Describe the timeline of the CNS’s development in relation to other systems and name four major structural CNS changes occurring in the fetus.
The nervous system is the first to begin developing and the last to finish.
- Cerebral hemisphere becomes the largest part of the brain
- Histological differentiation of cortex in the cerebrum and cerebellum
- Formation and myelination of nuclei and tracts
- Growth of spinal cord and vertebral column
When do sensory and motor systems develop? Which senses are the first to develop?
*discuss hearing, taste, vision, pain and movements
Sensory: hearing and taste mature before vision (organ of corti in the inner ear well developed at 5 months but retina still immature at birth).
Ascending tracts for pain are present but not myelinated at 19 weeks
Fetal movements can be detected by ultrasound at 8 weeks (and a repertoire of movements develop thereafter to help the baby “practice” for postnatal life, i.e suckling, breathing, etc) but not felt by mother until ~17 weeks
Name 5 tests that can monitor changes of the fetus’ anatomical and physiological parameters
- Ultrasound
- Fetal movements kick chart
- Doppler ultrasound
- Non-stress tests; NST monitors HR changes associated with fetal movements
- Biophysical profiles; includes 5 variables to be measured
When is a fetus regarded as having ‘growth restriction’? What are the two main types of growth restriction?
If their estimated weight is below the 10th percentile for their gestational age
- Symmetrical: growth restriction is generalized and proportional
- Asymmetrical: abdominal and head growth lags
How might you determine the duration of the pregnancy? (2 things)
Fertilization age, LMP
List 5 measurements and predictions that can be made in utero by ultrasound
- CRL
- Foot length
- Biparietal diameter
- Weight after delivery
- Appearance after delivery
What is the symphysis fundal height and what can it help determine? What similar measurement can alternatively be taken?
The distance between the symphysis pubis to the top of the uterus (i.e fundus). Alternatively, you can also asses the height of the fundus in relation to other structures such as the umbilicus or xiphisternum. A lag of 4cm or more of the fundal height suggests intrauterine growth restriction
What are four sources of variability for the symphysis fundal height?
- Number of fetuses
- The lie of the fetus
- Volume of amniotic fluid
- The extent of engagement of the head
Name the four histological stages of the development of the lungs and when they occur, briefly describe each.
- Pseudoglandular: 8-16 weeks, duct system begin to form up to terminal bronchioles
- Canalicular: 16th-26th week and is when the bronchioles form by budding off the pre-existing bronchiole structures formed during the pseudoglandular stage. Some terminal sacs, more vascular and pseudostratified ciliated columnar epithelia are forming
- Terminal Sac: 26 week- Term: Surfactant formed from week 20 (type 2 pneumocytes), many terminal sacs and some primitive alveoli
- Alveolar period: begins from last weeks of pregnancy to 8 years, 95% of alveoli formed post-natally
What happens to the fluid within the fetus’s lungs when it’s born?
The fluid is squeezed out at birth in normal vaginal delivery and any remaining is absorbed
What is the role of corticospinal tracts and when do they begin to develop? When does the brain start being myelinated?
Corticospinal tracts begin developing in the 4th month, they’re required for coordinated voluntary movements
9th month: Myelination of the brain
*both continue developing after birth
How might you clinically assess fetal urinary function?
Can use an ultrasound to check that the fetal bladder is emptying normally every 40-60 minutes
What is the role of surfactant?
Lowers the alveolar surface tension so that inspiration is made with less effort post-natally
Name three defects/disorders associated with polyhydramnios
Esophageal or duodenal atresia, CNS abnormalities
