Babies are not smaller. They are arriving earlier.
In "The averages moved. The structure did not." I reported that average birth weight fell 25 grams between 2016 and 2024.
Someone who read it asked whether that might simply be gestational age. Shorter pregnancies, smaller babies. It was a good question, and the answer was sitting in my own extract. I had never cut one against the other.
I did. They were right.
At every gestational age from 36 to 41 weeks, a baby in 2024 weighs almost exactly what it weighed in 2016. At 40 weeks the nine yearly averages span 8 grams. At 38 weeks, 7. At 39 weeks a baby in 2024 is 4 grams heavier than in 2016. These are averages over hundreds of thousands of births each.
What changed is how many babies are at each age. Births at 37 weeks went from 8.8 percent to 12.4, rising in every one of the nine years. Births at 40 weeks fell from 21.4 to 17.8, and at 41 weeks from 6.6 to 4.6.
I ran this on singleton births only. Twins and triplets are about 3 percent of the data and average nearly a kilogram lighter, so a small change in how many of them are born moves the national average on its own.
CDC WONDER rarely records whether a birth was induced. Where I did find it recorded, an induction that was medically necessary and one that was scheduled look identical.
Average birth weight is going down, and it is going down because pregnancies are getting shorter. The babies are the same size for their age.
Why pregnancies are getting shorter is not found in the CDC WONDER data.
The full data is shown in the following two tables.
What a baby weighs at a given gestational age
Singleton birth weights, average grams rounded to the gram, by year and the obstetric estimate of gestational age.
The range column is the distance between the highest and lowest of the nine years.
Read across any row. Nowhere does a row move by more than 14 grams in nine years, and no row has a trend.
| Weeks | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 | Range |
|---|---|---|---|---|---|---|---|---|---|---|
| 36 | 2,810 | 2,810 | 2,809 | 2,815 | 2,819 | 2,819 | 2,814 | 2,817 | 2,820 | 11 g |
| 37 | 3,021 | 3,020 | 3,024 | 3,026 | 3,028 | 3,024 | 3,014 | 3,018 | 3,020 | 14 g |
| 38 | 3,228 | 3,226 | 3,227 | 3,226 | 3,227 | 3,228 | 3,221 | 3,221 | 3,222 | 7 g |
| 39 | 3,409 | 3,408 | 3,412 | 3,411 | 3,414 | 3,419 | 3,410 | 3,411 | 3,414 | 11 g |
| 40 | 3,516 | 3,515 | 3,517 | 3,514 | 3,516 | 3,519 | 3,511 | 3,513 | 3,515 | 8 g |
| 41 | 3,630 | 3,629 | 3,631 | 3,629 | 3,632 | 3,638 | 3,630 | 3,636 | 3,638 | 9 g |
What pulled the average down
Share of single births at each gestational age, per cent of that year's single births.
37 weeks rises in every one of the nine years without a reversal, while 40 and 41 drain away. The babies are the same size for their age. There are more of the younger ones in the average.
| Weeks | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 |
|---|---|---|---|---|---|---|---|---|---|
| Under 37 | 8.02 | 8.13 | 8.24 | 8.47 | 8.42 | 8.76 | 8.67 | 8.71 | 8.76 |
| 37 | 8.77 | 9.16 | 9.58 | 10.13 | 10.54 | 11.23 | 11.63 | 12.05 | 12.43 |
| 38 | 16.30 | 16.43 | 16.60 | 16.86 | 16.92 | 17.30 | 17.45 | 17.58 | 17.64 |
| 39 | 38.54 | 38.47 | 38.73 | 39.51 | 40.03 | 39.61 | 38.84 | 38.75 | 38.53 |
| 40 | 21.39 | 21.00 | 20.44 | 19.40 | 18.80 | 18.10 | 18.26 | 17.94 | 17.77 |
| 41 | 6.61 | 6.47 | 6.11 | 5.36 | 5.04 | 4.76 | 4.88 | 4.70 | 4.61 |
| 42 or more | 0.36 | 0.34 | 0.31 | 0.26 | 0.25 | 0.25 | 0.27 | 0.27 | 0.26 |
Source: CDC WONDER Natality 2007-2024, filtered to 2016-2024, single births only, obstetric estimate of gestational age. Rows with unknown gestational age are excluded from the percentages.