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 23 2692 2759 6 lbs, 1 oz
35W,1D 25 2651 2756 6 lbs, 1 oz
35W,2D 24 2836 2746 6 lbs, 1 oz
35W,3D 33 2831 2748 6 lbs, 1 oz
35W,4D 21 2742 2780 6 lbs, 2 oz
35W,5D 33 2707 2765 6 lbs, 2 oz
35W,6D 24 2759 2779 6 lbs, 2 oz
36W,0D 51 2856 2794 35 g (1.2 oz) 6 lbs, 3 oz
36W,1D 42 2624 2813 57 g (2 oz) 6 lbs, 3 oz
36W,2D 54 2862 2859 113 g (4 oz) 6 lbs, 5 oz
36W,3D 49 2900 2915 167 g (5.9 oz) 6 lbs, 7 oz
36W,4D 46 2889 2938 158 g (5.6 oz) 6 lbs, 8 oz
36W,5D 59 3007 2971 206 g (7.3 oz) 6 lbs, 9 oz
36W,6D 74 3110 3011 232 g (8.2 oz) 6 lbs, 10 oz
37W,0D 118 2979 3033 239 g (8.4 oz) 6 lbs, 11 oz
37W,1D 114 2971 3063 250 g (8.8 oz) 6 lbs, 12 oz
37W,2D 114 3118 3091 232 g (8.2 oz) 6 lbs, 13 oz
37W,3D 118 3085 3097 182 g (6.4 oz) 6 lbs, 13 oz
37W,4D 143 3143 3140 202 g (7.1 oz) 6 lbs, 15 oz
37W,5D 137 3204 3171 200 g (7.1 oz) 6 lbs, 16 oz
37W,6D 179 3132 3188 177 g (6.2 oz) 7 lbs, 0 oz
38W,0D 248 3222 3221 188 g (6.6 oz) 7 lbs, 2 oz
38W,1D 214 3216 3253 190 g (6.7 oz) 7 lbs, 3 oz
38W,2D 218 3243 3268 177 g (6.2 oz) 7 lbs, 3 oz
38W,3D 258 3320 3301 204 g (7.2 oz) 7 lbs, 4 oz
38W,4D 333 3339 3324 184 g (6.5 oz) 7 lbs, 5 oz
38W,5D 312 3318 3349 178 g (6.3 oz) 7 lbs, 6 oz
38W,6D 358 3376 3362 174 g (6.1 oz) 7 lbs, 7 oz
39W,0D 518 3365 3384 163 g (5.7 oz) 7 lbs, 7 oz
39W,1D 454 3407 3400 147 g (5.2 oz) 7 lbs, 8 oz
39W,2D 413 3377 3413 145 g (5.1 oz) 7 lbs, 8 oz
39W,3D 553 3456 3433 132 g (4.7 oz) 7 lbs, 9 oz
39W,4D 553 3449 3462 138 g (4.9 oz) 7 lbs, 10 oz
39W,5D 534 3439 3483 134 g (4.7 oz) 7 lbs, 11 oz
39W,6D 585 3512 3504 142 g (5 oz) 7 lbs, 12 oz
40W,0D 1087 3512 3522 138 g (4.9 oz) 7 lbs, 12 oz
40W,1D 694 3559 3539 139 g (4.9 oz) 7 lbs, 13 oz
40W,2D 628 3568 3560 147 g (5.2 oz) 7 lbs, 14 oz
40W,3D 591 3598 3576 143 g (5 oz) 7 lbs, 14 oz
40W,4D 566 3596 3605 143 g (5 oz) 7 lbs, 15 oz
40W,5D 579 3614 3619 136 g (4.8 oz) 7 lbs, 16 oz
40W,6D 599 3638 3638 134 g (4.7 oz) 8 lbs, 0 oz
41W,0D 656 3663 3654 132 g (4.7 oz) 8 lbs, 1 oz
41W,1D 536 3658 3668 129 g (4.6 oz) 8 lbs, 1 oz
41W,2D 436 3719 3688 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 433 3719 3702 126 g (4.4 oz) 8 lbs, 3 oz
41W,4D 324 3715 3720 115 g (4.1 oz) 8 lbs, 3 oz
41W,5D 261 3794 3740 121 g (4.3 oz) 8 lbs, 4 oz
41W,6D 219 3731 3751 113 g (4 oz) 8 lbs, 4 oz
42W,0D 203 3788 3773 119 g (4.2 oz) 8 lbs, 5 oz
42W,1D 116 3783 3795 127 g (4.5 oz) 8 lbs, 6 oz
42W,2D 68 3857 3799 111 g (3.9 oz) 8 lbs, 6 oz
42W,3D 48 4032 3831 129 g (4.6 oz) 8 lbs, 7 oz
42W,4D 19 3891 3849 129 g (4.6 oz) 8 lbs, 8 oz
42W,5D 25 3877 3890 150 g (5.3 oz) 8 lbs, 9 oz
42W,6D 12 3898 3908 157 g (5.5 oz) 8 lbs, 10 oz
Total responses: 15150

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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