HUMAN BIRTH WEIGHT
1. Variation in birth weight
2. Sources of variation in birth weight
3. Implications of variation in birth weight
Normal Rate of Fetal Growth
•
•
•
•
•
15 weeks
20 weeks
30 weeks
35 weeks
40 weeks
=
=
=
=
=
5 grams/day
10 grams/day
25 grams/day
35 grams/day
15 grams/day
– May vary by race, gender, multiple gestation
Means:
Males: 3625 gms 7lbs 10oz
Females: 3472 gms 7lbs 6oz
Categorizing birth weight
Before 1960s:
1. Low Birth Weight or LBW defined as less than 2500 gms
=Prematurity
2. All other babies 2500 gms and over
3. Focus was on low birth weight babies because of
their increased mortality
Low Birth Weight
• Low Birthweight
– <2500 grams
• Often due to prematurity
• Very Low Birthweight
– <1500 grams
• Almost always a prematurity component
• Differences in outcome if LBW due to:
– Prematurity
– Term but SGA
– Preterm plus SGA
Problems with this classification:
1. Not all small babies are premature
2. Not all premature babies are small
World Health Organization 1961 promoted use of
“preterm” rather than “premature”
Gestation: time from fertilization to delivery
normally 38 weeks (266 days).
▪Large for gestational age: Weight is above the 90th percentile
at gestational age (LGA) > 8lbs 3oz (4000gm)
▪Appropriate for gestational age: Normal birth weight (AGA)
▪Small for gestational age: Weight is below the 10th percentile at
gestational age (SGA) < 5lbs 7oz ( 2600gm)
High Birth Weight
• Incidence: until the new WHO sponsored
standard neonatal growth charts become
available in 2014, estimated frequencies
indicate that HBW represents anywhere
from 3,5% to 10% of all births.
• This large variation is probably due to
changing definitions applied to different
populations.
HUMAN BIRTH WEIGHT
1. Variation in birth weight
2. Sources of variation in birth weight
3. Implications of variation in birth weight
High Birth Weight
• Large babies can be born under two
different circumstances:
• Healthy mothers, who are themselves
constitutionally large, often married to
large fathers.
High Birth Weight
• Mothers who have either metabolic or
genetic problems: most common are the
disorders
related
to
carbohydrate
intolerance during pregnancy:
• insulin-dependent diabetes;
• gestational diabetes,
• metabolic syndrome;
• maternal obesity.
High Birth Weight
Influence of Maternal Weight Gain during Pregnancy
Influence of altitude on birth weight: Colorado
Increase in
birth weight
with increased
parity
Plurality (Multiple births)
Singleton birth weight
Twin birth weight
Triplet birth weight
(LBW 5lb 8oz (2494gm))
7lbs 6oz (3356gm)
5lbs 10oz (2585gm)
4lbs 4oz (1911gm)
Dizygotic twins with
two amnions, two chorions
or
Monozygotic twins with
Two amnions, two chorions
Monozygotic twins with
Two amnions, one chorion
Higher >
Lower
Monozygotic twins with
One amnion, one chorion
Sex of the twins
Same sex: Higher concordance in birth weight
Opposite sex: Higher discordance in birth weight
DZ twins
MM
MF Female twins larger on average than FF twins
FF
Sex of the twins
Same sex: Higher concordance in birth weight
Opposite sex: Higher discordance in birth weight
Higher prenatal mortality
Whites
African American
Hispanic
Native American
3549
3330
3452
3523
Heritability of birth weight
Adjusted for sex and term
74%
Effect of environment: First generation of immigrants
Asian immigrant study
Born in Asia:
2946g
First generation UK:
3196g
Gestational diabetes (GDM) in the mother
increases
1. The risk of a high birth weight baby
(LGA)
2. The risk of GDM in subsequent pregnancies
Factors that can influence birth weight:
Altitude
Sex
Parity
Multiple or single birth
Genetics
Maternal health (GDM)
Maternal nutrition (excessive or insufficient weight gain)
Nutritional Etiologies
• Malnutrition must be severe to have a
dramatic effect
– Leningrad: prolonged malnutrition:
• BW reduced 400-600 grams
– Holland: 6 months of famine
• BW reduced 10% if famine during 3rd trimester
• Depends on preceding nutritional status and
timing of malnutrition
HUMAN BIRTH WEIGHT
1. Variation in birth weight
2. Sources of variation in birth weight
3. Implications of variation in birth weight
Epidemiological studies revealed that full-term babies
with low birth weight had higher mortality than full
term-babies of normal weight.
New terminology:
Small for Gestational Age (SGA) (small babies who are
not preterm, just below the 10 percentile for GA)
Intrauterine Growth Retardation (IUGR)
Manning, 1995
Combined Term Neonatal Outcomes
by Birth Weight Range
80
70
Percent
60
50
40
Meconium, "Fetal
Distress," or Low
5min Apgar
NICU/Neonatal
Complication
Any Perinatal
Complication
30
20
10
0
>3000g
2500-3000g
2000-2500g
<2000g
IUGR Mortality
• Perinatal mortality inversely proportional to
percentile
– 0-5x relative risk if between 2.5th-10th %
– 8-10x relative risk if <2.5th %
• Fetal mortality is 50% higher than neonatal
– Males higher than females
• Growth versus Gestational Age
High Birth Weight
• A potential risk associated to high birth
weight is the higher chances to develop
leukemia. Drs. Robert Caughey and Karin
Michels form Harvard published in the
International Journal of Cancer (2009) their
results: an overall risk of developing
leukemia 35 times higher for higher birth
weight babies than for those of normal birth
weight.
High Birth Weight
Fifteen-year expected adult weight trajectories by birth weight group in Michigan Bone Health and
Metabolism Study women. Normal, low, and high birth weight was defined as 2,500–4,000 g, <2,500 g, and
>4,000 g, respectively. Fifteen-year trajectories were adjusted for participant's age at baseline and were
similar in all other body composition measures, except waist-to-hip ratio. Eileen Rillamas-Sun, MaryFran
R. Sowers, Siobán D. Harlow and John F. Randolph Jr. Obesity 20, 463-465 (February 2012)
High Birth Weight
Conclusions:
Babies born with HBW represent a smaller group than
those born with low birth weight.
They are, nonetheless , an important public health issue
because:
1-They have immediate problems at birth.
2-They are now the focus of larger epidemiological
investigations, because long term effects of neonatal
macrosomia do not fade; rather, they persist through
lifetime increasing the risk of developing obesity,
metabolic syndrome, diabetes mellitus and hypertensive
disorders.
OTHER SPECIES:
Dairy Cattle
Meta-analysis of research projects from across country:
Birth weight was most consistently important trait
for explaining variability in milk production
Heifers weighing between 30-40 kg (66-88 lbs)
produced most milk as cows