The National Center for Health Statistics (NCHS) keeps records of many health-related aspects of people, including the birth weights of all babies born in the United States.
The birth weight of a baby is related to its gestation period (the time between conception and birth). For a given gestation period, the birth weights can be approximated by a normal distribution. The means and standard deviations of the birth weights for various gestation periods are shown in the table below.
One of the many goals of the NCHS is to reduce the percentage of babies born with low birth weights. The figure below shows the percents of preterm births and low birth weights from 2011 to 2019.
Gestation
period
Mean birth Standard
weight
deviation
Under 28 weeks
1.90 lb
0.72 lb
28 to 31 weeks
3.49 lb
1.01 lb
32 to 33 weeks
4.62 lb
0.95 lb
34 to 36 weeks
5.97 lb
1.02 lb
37 to 38 weeks
7.01 lb
0.92 lb
39 to 40 weeks
7.59 lb
0.84 lb
41 weeks
7.92 lb
0.87 lb
42 weeks and over
8.07 lb
1.01 lb
1. The distributions of birth weights for three gestation periods are shown. Match each curve with a gestation period. Explain your reasoning.
(a)
(b)
(c)
2. What percent of the babies born within each gestation period have a low birth weight (under 5.5 pounds)?
(a) 28 to 31 weeks
(c) 39 to 40 weeks
(b) 32 to 33 weeks
(d) 42 weeks and over
3. Describe the weights of the top 10% of the babies born within each gestation period.
(a) Under 28 weeks
(c) 41 weeks
(b) 34 to 36 weeks
(d) 42 weeks and over
4. For each gestation period, what is the probability that a baby will weigh between 6 and 9 pounds at birth?
(a) Under 28 weeks
(c) 37 to 38 weeks
(b) 32 to 33 weeks
(d) 41 weeks
5. A birth weight of less than 3.25 pounds is classified by the NCHS as a "very low birth weight." What is the probability that a baby has a very low birth weight for each gestation period?
(a) Under 28 weeks
(c) 32 to 33 weeks
(b) 28 to 31 weeks
(d) 39 to 40 weeks
Preterm = under 37 weeks
Low birth weight = under 5.5 pounds
Percent of preterm births
Percent of low birth weights
Year
