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 2775 6 lbs, 2 oz
36W,0D 50 2859 2789 34 g (1.2 oz) 6 lbs, 2 oz
36W,1D 41 2621 2809 57 g (2 oz) 6 lbs, 3 oz
36W,2D 50 2845 2856 113 g (4 oz) 6 lbs, 5 oz
36W,3D 48 2893 2915 170 g (6 oz) 6 lbs, 7 oz
36W,4D 43 2897 2940 166 g (5.9 oz) 6 lbs, 8 oz
36W,5D 56 3015 2970 212 g (7.5 oz) 6 lbs, 9 oz
36W,6D 73 3114 3011 236 g (8.3 oz) 6 lbs, 10 oz
37W,0D 116 2983 3034 245 g (8.6 oz) 6 lbs, 11 oz
37W,1D 108 2957 3064 255 g (9 oz) 6 lbs, 12 oz
37W,2D 109 3122 3091 235 g (8.3 oz) 6 lbs, 13 oz
37W,3D 117 3085 3095 180 g (6.3 oz) 6 lbs, 13 oz
37W,4D 137 3146 3138 198 g (7 oz) 6 lbs, 15 oz
37W,5D 132 3207 3170 200 g (7.1 oz) 6 lbs, 16 oz
37W,6D 172 3121 3187 176 g (6.2 oz) 7 lbs, 0 oz
38W,0D 234 3225 3221 187 g (6.6 oz) 7 lbs, 2 oz
38W,1D 206 3212 3252 188 g (6.6 oz) 7 lbs, 3 oz
38W,2D 212 3244 3266 175 g (6.2 oz) 7 lbs, 3 oz
38W,3D 251 3322 3301 206 g (7.3 oz) 7 lbs, 4 oz
38W,4D 326 3334 3322 184 g (6.5 oz) 7 lbs, 5 oz
38W,5D 303 3315 3347 177 g (6.2 oz) 7 lbs, 6 oz
38W,6D 350 3380 3360 173 g (6.1 oz) 7 lbs, 7 oz
39W,0D 506 3357 3380 159 g (5.6 oz) 7 lbs, 7 oz
39W,1D 446 3405 3397 145 g (5.1 oz) 7 lbs, 8 oz
39W,2D 406 3376 3412 146 g (5.1 oz) 7 lbs, 8 oz
39W,3D 537 3449 3431 130 g (4.6 oz) 7 lbs, 9 oz
39W,4D 542 3450 3462 140 g (4.9 oz) 7 lbs, 10 oz
39W,5D 523 3441 3483 136 g (4.8 oz) 7 lbs, 11 oz
39W,6D 569 3515 3504 144 g (5.1 oz) 7 lbs, 12 oz
40W,0D 1052 3515 3522 142 g (5 oz) 7 lbs, 12 oz
40W,1D 674 3555 3540 143 g (5 oz) 7 lbs, 13 oz
40W,2D 614 3568 3560 148 g (5.2 oz) 7 lbs, 14 oz
40W,3D 568 3598 3575 144 g (5.1 oz) 7 lbs, 14 oz
40W,4D 552 3596 3603 141 g (5 oz) 7 lbs, 15 oz
40W,5D 572 3614 3618 135 g (4.8 oz) 7 lbs, 16 oz
40W,6D 589 3633 3638 134 g (4.7 oz) 8 lbs, 0 oz
41W,0D 638 3662 3653 131 g (4.6 oz) 8 lbs, 1 oz
41W,1D 523 3660 3668 128 g (4.5 oz) 8 lbs, 1 oz
41W,2D 419 3725 3688 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 416 3721 3702 127 g (4.5 oz) 8 lbs, 3 oz
41W,4D 315 3714 3720 117 g (4.1 oz) 8 lbs, 3 oz
41W,5D 250 3792 3740 122 g (4.3 oz) 8 lbs, 4 oz
41W,6D 214 3719 3748 110 g (3.9 oz) 8 lbs, 4 oz
42W,0D 199 3790 3769 116 g (4.1 oz) 8 lbs, 5 oz
42W,1D 114 3773 3791 123 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 3830 128 g (4.5 oz) 8 lbs, 7 oz
42W,4D 19 3891 3846 126 g (4.4 oz) 8 lbs, 8 oz
42W,5D 23 3909 3890 150 g (5.3 oz) 8 lbs, 9 oz
42W,6D 12 3898 3914 166 g (5.9 oz) 8 lbs, 10 oz
Total responses: 14735

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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