Gestation vs. Birthweight

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Birthweight (in grams) vs. Gestation time

The CDC has better data on this, but pregnant women frequently wonder whether their late babies will be huge so I asked my survey participants for weight just for funzies. On average, babies born later are slightly bigger but not much. The difference between a baby born around 39W 0D (about 3400 g) and one born around 41 W 0D (about 3700 g) is only 300 g, or about 10.6 ounces.

There's a lot of variety too; plenty of babies born after 41 weeks weigh less than babies born around week 38 or even 37. It should be no surprise that they all just seem to grow at their own rate.


Data table

Date# BirthsAverage (g)± 3 day Average (g)Change from previous week± 3 day Average (lbs oz)
35W,0D 20 2735 2772 6 lbs, 2 oz
35W,1D 23 2652 2767 6 lbs, 2 oz
35W,2D 24 2836 2754 6 lbs, 1 oz
35W,3D 33 2831 2755 6 lbs, 1 oz
35W,4D 21 2742 2782 6 lbs, 2 oz
35W,5D 33 2707 2766 6 lbs, 2 oz
35W,6D 24 2759 2778 6 lbs, 2 oz
36W,0D 50 2859 2792 20 g (0.7 oz) 6 lbs, 2 oz
36W,1D 42 2624 2810 43 g (1.5 oz) 6 lbs, 3 oz
36W,2D 51 2858 2857 103 g (3.6 oz) 6 lbs, 5 oz
36W,3D 48 2893 2914 159 g (5.6 oz) 6 lbs, 7 oz
36W,4D 46 2889 2939 157 g (5.5 oz) 6 lbs, 8 oz
36W,5D 59 3007 2970 204 g (7.2 oz) 6 lbs, 9 oz
36W,6D 73 3114 3010 232 g (8.2 oz) 6 lbs, 10 oz
37W,0D 116 2983 3033 241 g (8.5 oz) 6 lbs, 11 oz
37W,1D 111 2961 3063 253 g (8.9 oz) 6 lbs, 12 oz
37W,2D 112 3120 3091 234 g (8.3 oz) 6 lbs, 13 oz
37W,3D 118 3085 3096 182 g (6.4 oz) 6 lbs, 13 oz
37W,4D 139 3146 3138 199 g (7 oz) 6 lbs, 15 oz
37W,5D 132 3207 3169 199 g (7 oz) 6 lbs, 16 oz
37W,6D 176 3127 3187 177 g (6.2 oz) 7 lbs, 0 oz
38W,0D 237 3223 3221 188 g (6.6 oz) 7 lbs, 2 oz
38W,1D 207 3211 3253 190 g (6.7 oz) 7 lbs, 3 oz
38W,2D 213 3245 3268 177 g (6.2 oz) 7 lbs, 3 oz
38W,3D 253 3323 3302 206 g (7.3 oz) 7 lbs, 4 oz
38W,4D 328 3337 3324 186 g (6.6 oz) 7 lbs, 5 oz
38W,5D 307 3317 3349 180 g (6.3 oz) 7 lbs, 6 oz
38W,6D 355 3380 3362 175 g (6.2 oz) 7 lbs, 7 oz
39W,0D 511 3361 3382 161 g (5.7 oz) 7 lbs, 7 oz
39W,1D 451 3407 3399 146 g (5.1 oz) 7 lbs, 8 oz
39W,2D 409 3376 3413 145 g (5.1 oz) 7 lbs, 8 oz
39W,3D 545 3452 3433 131 g (4.6 oz) 7 lbs, 9 oz
39W,4D 548 3450 3462 138 g (4.9 oz) 7 lbs, 10 oz
39W,5D 529 3441 3483 134 g (4.7 oz) 7 lbs, 11 oz
39W,6D 578 3514 3504 142 g (5 oz) 7 lbs, 12 oz
40W,0D 1075 3513 3523 141 g (5 oz) 7 lbs, 12 oz
40W,1D 684 3558 3540 141 g (5 oz) 7 lbs, 13 oz
40W,2D 623 3568 3560 147 g (5.2 oz) 7 lbs, 14 oz
40W,3D 582 3601 3576 143 g (5 oz) 7 lbs, 14 oz
40W,4D 558 3597 3605 143 g (5 oz) 7 lbs, 15 oz
40W,5D 575 3613 3619 136 g (4.8 oz) 7 lbs, 16 oz
40W,6D 595 3636 3639 135 g (4.8 oz) 8 lbs, 0 oz
41W,0D 645 3663 3654 131 g (4.6 oz) 8 lbs, 1 oz
41W,1D 530 3658 3669 129 g (4.6 oz) 8 lbs, 1 oz
41W,2D 428 3724 3688 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 427 3720 3702 126 g (4.4 oz) 8 lbs, 3 oz
41W,4D 320 3716 3720 115 g (4.1 oz) 8 lbs, 3 oz
41W,5D 254 3790 3740 121 g (4.3 oz) 8 lbs, 4 oz
41W,6D 216 3722 3748 109 g (3.8 oz) 8 lbs, 4 oz
42W,0D 201 3788 3769 115 g (4.1 oz) 8 lbs, 5 oz
42W,1D 115 3775 3791 122 g (4.3 oz) 8 lbs, 6 oz
42W,2D 67 3847 3794 106 g (3.7 oz) 8 lbs, 6 oz
42W,3D 48 4032 3828 126 g (4.4 oz) 8 lbs, 7 oz
42W,4D 19 3891 3844 124 g (4.4 oz) 8 lbs, 8 oz
42W,5D 25 3877 3886 146 g (5.1 oz) 8 lbs, 9 oz
42W,6D 12 3898 3908 160 g (5.6 oz) 8 lbs, 10 oz
Total responses: 14939

Due Date Survey Data

Due date statistics: A study on the length of pregnancy
Probability of delivery resulting from spontaneous labor after 35 weeks
Probability of delivery within x days of a given date
Length of pregnancy by week
Spontaneous labor and due date determination
Length of pregnancy, comparing subsequent births for individual moms
Length of pregnancy for first time vs. second & third time moms
Length of pregnancy, type of delivery
Gestation vs. Birthweight
Probablity of Induction after a given day
Average day of spontaneous labor vs. age of mother at time of birth
Are more babies born during a full moon?
What's the most common day of the week for babies to be born?
I'm still pregnant at 40W. What's it mean?
Do winter babies arrive later?
What if I know my conception date?
Are boys or girls born later?
Subsequent pregnancy date search
How do inductions bias the due date statistics?
Download the data
Birth Stories
Survey input dates

More Resources

Pregnancy Day-by-Day
Fast pregnancy calendar
Baby age calendar
EDD Icon Generator
Jo's Birth Story
Weeks vs. Months Explained
Charts generated by flot
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