HUMAN BIRTH WEIGHT 1.Variation in birth weight 2.Sources of variation in birth weight 3.Implications...
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HUMAN BIRTH WEIGHT
1. Variation in birth weight
2. Sources of variation in birth weight
3. Implications of variation in birth wieght
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Males: 3625 gms 7lbs 10ozFemales: 3472 gms 7lbs 6oz
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Categorizing birth weight
Before 1960s
1. Low Birth Weight or LBW less than 2500gms
= Prematurity
2. All other babies 2500 gms and over
3. Focus was on low birth weight babies because of
their increased mortality
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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 deliverynormally 38 weeks (266 days).
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Epidemiological studies revealed that full-term babieswith low birth weight had higher mortality than full term-babies of normal weight.
New terminology:
Small for Gestational Age (SGA) (small babies who arenot preterm, just below the 10 percentile for GA)
Intrauterine Growth Retardation (IUGR)
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▪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)
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HUMAN BIRTH WEIGHT
1. Variation in birth weight
2. Sources of variation in birth weight
3. Implications of variation in birth weight
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Influence of altitude on birth weight: Colorado
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Increase in birth weight with increasedparity
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Singleton birth weight 7lbs 6oz (3356gm)Twin birth weight 5lbs 10oz (2585gm) Triplet birth weight 4lbs 4oz (1911gm)
(LBW 5lb8oz (2494gm))
Plurality (Multiple births)
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Zygosity and placentation
Zygosity: MZ or DZPlacentation: Features of the placenta
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Monozygotic twins withOne amnion, one chorion
Monozygotic twins withTwo amnions, one chorion
Dizygotic twins withtwo amnions, two chorions
orMonozygotic twins withTwo amnions, two chorions
Higher > Lower
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Sex of the twins
Same sex: Higher concordance in birth weight
Opposite sex: Higher discordance in birth weight
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DZ twins
MM
MF Female twins larger on average than FF twins
FF
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Sex of the twins
Same sex: Higher concordance in birth weight
Opposite sex: Higher discordance in birth weight Higher prenatal mortality
Associated with discordance for weight?
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Heritability of birth weight 74%
Adjusted for sex and term
Whites 3549African American 3330Hispanic 3452Native American 3523
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Effect of environment: First generation of immigrants
Asian immigrant study
Born in Asia: 2946g
First generation UK: 3196g
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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
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Factors that can influence birth weight:
Altitude
Sex
Parity
Multiple or single birth
Genetics
Maternal health (GDM)
Maternal nutrition (excessive or insufficient weight gain)
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HUMAN BIRTH WEIGHT
1. Variation in birth weight
2. Sources of variation in birth weight
3. Implications of variation in birth weight