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 2755 6 lbs, 1 oz
35W,1D 23 2652 2752 6 lbs, 1 oz
35W,2D 22 2766 2743 6 lbs, 1 oz
35W,3D 33 2831 2745 6 lbs, 1 oz
35W,4D 21 2742 2774 6 lbs, 2 oz
35W,5D 33 2707 2758 6 lbs, 1 oz
35W,6D 24 2759 2778 6 lbs, 2 oz
36W,0D 50 2859 2792 37 g (1.3 oz) 6 lbs, 2 oz
36W,1D 42 2624 2810 58 g (2 oz) 6 lbs, 3 oz
36W,2D 51 2858 2857 114 g (4 oz) 6 lbs, 5 oz
36W,3D 48 2893 2914 169 g (6 oz) 6 lbs, 7 oz
36W,4D 46 2889 2939 165 g (5.8 oz) 6 lbs, 8 oz
36W,5D 59 3007 2970 212 g (7.5 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 175 3126 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 206 3212 3253 190 g (6.7 oz) 7 lbs, 3 oz
38W,2D 212 3244 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 3325 187 g (6.6 oz) 7 lbs, 5 oz
38W,5D 306 3318 3350 181 g (6.4 oz) 7 lbs, 6 oz
38W,6D 354 3382 3362 175 g (6.2 oz) 7 lbs, 7 oz
39W,0D 511 3361 3383 162 g (5.7 oz) 7 lbs, 7 oz
39W,1D 450 3407 3400 147 g (5.2 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 547 3451 3463 138 g (4.9 oz) 7 lbs, 10 oz
39W,5D 527 3441 3483 133 g (4.7 oz) 7 lbs, 11 oz
39W,6D 576 3515 3504 142 g (5 oz) 7 lbs, 12 oz
40W,0D 1067 3514 3523 140 g (4.9 oz) 7 lbs, 12 oz
40W,1D 683 3558 3540 140 g (4.9 oz) 7 lbs, 13 oz
40W,2D 622 3567 3560 147 g (5.2 oz) 7 lbs, 14 oz
40W,3D 580 3600 3576 143 g (5 oz) 7 lbs, 14 oz
40W,4D 556 3598 3605 142 g (5 oz) 7 lbs, 15 oz
40W,5D 573 3614 3619 136 g (4.8 oz) 7 lbs, 16 oz
40W,6D 593 3635 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 527 3656 3668 128 g (4.5 oz) 8 lbs, 1 oz
41W,2D 426 3724 3688 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 426 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 253 3790 3740 121 g (4.3 oz) 8 lbs, 4 oz
41W,6D 216 3722 3749 110 g (3.9 oz) 8 lbs, 4 oz
42W,0D 200 3791 3770 116 g (4.1 oz) 8 lbs, 5 oz
42W,1D 115 3775 3791 123 g (4.3 oz) 8 lbs, 6 oz
42W,2D 67 3847 3796 108 g (3.8 oz) 8 lbs, 6 oz
42W,3D 48 4032 3831 129 g (4.6 oz) 8 lbs, 7 oz
42W,4D 19 3891 3847 127 g (4.5 oz) 8 lbs, 8 oz
42W,5D 24 3909 3890 150 g (5.3 oz) 8 lbs, 9 oz
42W,6D 12 3898 3914 165 g (5.8 oz) 8 lbs, 10 oz
Total responses: 14899

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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