5 - Fetal Growth Flashcards
Define fetal growth
FETAL GROWTH
The increase in mass that occurs between the end of the embryonic period and birth.
What methods were used to assess fetal growth historically?
EXTERNAL DETERMINATION OF SIZE
Determine the size of the infant by palpation of the maternal abdomen
This is the basis of determination of the Symphysis Fundal Height (SPH)
This identifies the distance between the pubic symphysis and the top of the uterus
Values that are lower than they should may result from: wrong last menstrual period date, the baby in a transverse lie, or complications including oligohydramnios (low levels of amniotic fluid) or a baby that is small for gestational age (SGA)
Higher values may also be found, due to: wrong last menstrual period date, multiple pregnancy, or maternal obesity.
Complications could include molar pregnancy, fibroids, polyhydramnios or a baby that is large for gestational age (LGA).
This simple and inexpensive measurement may identify gross changes in size, and hence gross complications in the pregnancy, but is generally of limited use, thanks to the many confounders, which include the problems listed above, as well as considerable inter-operator variability.
Outline historical data regarding fetal size
Initial information on the actual size of the fetus, and hence on fetal growth was obtained from miscarriages of pregnancy (Figure 6.3). While the did give information of interest, this did not take account of the possible causative relationship between low fetal growth leading to miscarriage, and hence such data may be inaccurate.
It can be seen that fetal weight continues to increase during pregnancy, while fetal length changes less in the later stages.
Such summaries have now been replaced by data from in utero scanning, and these will be used in the rest of this chapter.
What are the two primary components responsible for the extent of fetal growth?
1. GENETIC POTENTIAL
- this is derived from both parents
- reflects the logical view that parents who are taller or bigger will have infants that are different in size to parents who are shorter or lighter in build
- this will be mediated by factors under genetic control, including mediators such as the insulin-like growth factors.
2. SUBSTRATE SUPPLY
- sufficient nutrients are essential to achieve genetic potential
- this is primarily based on the placenta which is dependent upon both uterine and placental vascularity.
Define ‘Small for Gestational Age/Growth Restricted’
Infant born at term, weighing about 1000 grams or less
By what biometrical parameters is fetal growth assessed?
Fetal growth is assessed by 4 biometrical parameters:
- Biparietal diameter (BPD)
- Head Circumference (HC)
- Abdominal Circumference (AC)
- Femur Length (FL)
They are combined to give the Estimated Fetal Weight (EFW).
How have normative growth curves been constructed?
Using the 4 biometric parameters of growth
These growth curves are expressed in centiles
What are normative growth curves used for?
Used to clinically identify normal intrauterine growth and detect risk of obstetric and neonatal complications
Normally, it is sequential measurements that are important and not a single measurement
What could be a considered more important use of ultrasound technology in pregnancy, rather than montioring fetal growth?
Can be used to assess fetal wellbeing
Outline what typical ultrasounds would show
Why is the use of centiles important on growth curves?
The use of centiles is important because this allows compensation for different sizes of infants that are growing and developing normally
What are the underlying principals behind using customised fetal growth charts for a more individualised assessment?
They are based on fetal weight curves for normal pregnancies.
The are adjusted to reflect maternal constitutional variation e.g. maternal height, weight, ethnicity, parity.
They are optimised by presenting a standard free from pathological factors such as diabetes and smoking.
What is the average rate at which an infant gains weight?
14-15 wks: 5g /day
20 wks: 10 g/day
32-34 wks: 30-35g/day
>34 wks: growth rate decreases
Note that the final point, of decreased velocity towards the end of pregnancy, does not agree well with the early data in Figure 6.3, showing that the miscarriage data is not reliable.
What three phases characterise normal fetal growth?
Cellular hyperplasia (increased cell numbers): 4-20 weeks
Hyperplasia and hypertrophy (increased cell size): 20-28 weeks
Hypertrophy dominates: 28-40 weeks
As the main increase in fetal weight occurs during the final trimester of pregnancy, hypertrophy is a key facto
Why is the Last Menstrual Period (LMP) dating of pregnancy not reliable?
CONFOUNDING FACTORS:
- Irregular length of periods
- abnormal endometrial bleeding
- use of oral contraceptives
- breastfeeding
There are also other practicalities, in that a couple hoping to start a family are likely to take careful note of LMP timings, whereas in the case of an unplanned pregnancy, maternal information on her last menstrual period may not be so precise.
Why is correct dating of pregnancies so important?
Correct dating is very important, as a change in the dates may lead to a pregnancy being inappropriately identified as Large or Small for gestational age.
Clinical decisions about delivery timings and methods (induction or Caesarean section) may not be correct; glucocorticoids are given prior to preterm delivery to enhance lung surfactant production and subsequent lung function.
What is best practice for dating a pregnancy?
Best practice is therefore to date the pregnancy by ultrasound, determining the Crown-Rump length of the fetus, preferably towards the end of the first trimester
Variations in fetal size are more limited at this stage of development, so the gestational age of the infant can be estimated more precisely.
List maternal factors that can influence fetal growth
Poverty
Mother’s Age
Drug Use
Alcohol
Smoking and Nicotine
Diseases
Mother’s Diet and Physical Health
Mother’s Prenatal Depression
Environmental Toxins
How can maternal poverty influence fetal growth?
POVERTY
Has been linked to poor prenatal care and has been an influence on prenatal development
Women in poverty are more likely to have children at a younger age, which can result in low birth weight.
Many of these expecting mothers have little education and are therefore less aware of the risks of smoking, alcohol, and drugs – other factors that influence the growth rate of a fetus.
Women in poverty are more likely to have diseases that are harmful to the fetus.
How can maternal age influence fetal growth?
MATERNAL AGE
Women between the ages of 16 and 35 have a healthier environment for a fetus than women under 16 or over 35.
Women between this age gap are more likely to have fewer complications.
Women over 35 are more inclined to have a longer labour period, which could potentially result in death of the mother or fetus.
Women under 16 and over 35 have a higher risk of preterm labour (premature baby), and this risk increases for women in poverty, African Americans, and women who smoke.
Young mothers are more likely to engage in high risk behaviors, such as using alcohol, drugs, or smoking, resulting in negative consequences for the fetus.
Premature babies from young mothers are more likely to have neurological defects that will influence their coping capabilities – irritability, trouble sleeping, constant crying for example.
There is increased risk of Down syndrome for infants born to those aged over 40 years.
Young teenaged mothers (younger than 16), and mothers over 35, are more exposed to the risks of miscarriages, premature births, and birth defects.
How can maternal drug use influence fetal growth?
DRUG USE
11% of fetuses are exposed to illicit drug use during pregnancy.
Maternal drug use occurs when drugs ingested by the pregnant woman are metabolized in the placenta and then transmitted to the fetus.
When using drugs (narcotics), there is a greater risk of birth defects, low birth weight, and a higher rate of death in infants or stillbirths.
Drug use may lead to extreme irritability, crying, and risk for SIDS once the fetus is born.
The chemicals in drugs can cause an addiction in the babies once they are born.
Marijuana will slow the fetal growth rate and can result in premature delivery. It can also lead to low birth weight, a shortened gestational period and complications in delivery.
Heroin will cause interrupted fetal development, stillbirths, and can lead to numerous birth defects. Heroin can also result in premature delivery, creates a higher risk of miscarriages, result in facial abnormalities and head size, and create gastrointestinal abnormalities in the fetus. There is an increased risk for SIDS, dysfunction in the central nervous system, and neurological dysfunctions including tremors, sleep problems, and seizures. The fetus is also put at a great risk for low birth weight and respiratory problems.
Cocaine use results in a smaller brain, which results in learning disabilities for the fetus. Cocaine puts the fetus at a higher risk of being stillborn or premature. Cocaine use also results in low birthweight, damage to the central nervous system, and motor dysfunction.
How do you choose which centile to use on a birthweight plot?
When choosing which centile to use, a balance between sensitivity and specificity is being made – the tenth centile is most sensitive and the third centile is most specific
The tenth centile will capture all babies with IUGR, but will also include those babies that are just small for gestational age, i.e. you get a number of false positives.
All babies recorded using the third centile will have IUGR, but some IUGR babies may be missed, i.e you get a number of false negatives.
Note that none of these measurements take parental characteristics into account, so there is no consideration of the importance of genetic factors.
Summarise the overlaps between centile weights and actual weights
How can growth be assessed?
Growth is a dynamic process of a change of size over time and, therefore, it can only be assessed by serial observation.
The serial measurements show that the infant is not growing ‘along a centile’, but is growing less than would be expected.
What is the link between IUGR and pre-eclampsia?
IUGR AND PRE-ECLAMPSIA
There is a close link between pre-eclampsia (pre-eclamptic toxaemia, PET) and IUGR, so that these complications often occur together.
The reason seems to be that the main cause of pre-eclampsia is diminished remodelling of the spiral arteries by cytotrophoblast (Figure 6.13), which causes decreased blood flow and hence decreased nutrient supply to the placenta and fetus.
