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 3172 201 g (7.1 oz) 6 lbs, 16 oz
37W,6D 179 3132 3189 178 g (6.3 oz) 7 lbs, 0 oz
38W,0D 248 3222 3222 189 g (6.7 oz) 7 lbs, 2 oz
38W,1D 218 3220 3253 190 g (6.7 oz) 7 lbs, 3 oz
38W,2D 218 3243 3269 178 g (6.3 oz) 7 lbs, 3 oz
38W,3D 258 3320 3301 204 g (7.2 oz) 7 lbs, 4 oz
38W,4D 333 3339 3325 185 g (6.5 oz) 7 lbs, 5 oz
38W,5D 312 3318 3350 178 g (6.3 oz) 7 lbs, 6 oz
38W,6D 361 3377 3363 174 g (6.1 oz) 7 lbs, 7 oz
39W,0D 519 3367 3384 162 g (5.7 oz) 7 lbs, 7 oz
39W,1D 456 3406 3400 147 g (5.2 oz) 7 lbs, 8 oz
39W,2D 414 3378 3414 145 g (5.1 oz) 7 lbs, 8 oz
39W,3D 555 3456 3434 133 g (4.7 oz) 7 lbs, 9 oz
39W,4D 553 3449 3462 137 g (4.8 oz) 7 lbs, 10 oz
39W,5D 535 3440 3483 133 g (4.7 oz) 7 lbs, 11 oz
39W,6D 586 3512 3504 141 g (5 oz) 7 lbs, 12 oz
40W,0D 1090 3513 3522 138 g (4.9 oz) 7 lbs, 12 oz
40W,1D 696 3559 3539 139 g (4.9 oz) 7 lbs, 13 oz
40W,2D 633 3568 3560 146 g (5.1 oz) 7 lbs, 14 oz
40W,3D 593 3599 3576 142 g (5 oz) 7 lbs, 14 oz
40W,4D 568 3595 3605 143 g (5 oz) 7 lbs, 15 oz
40W,5D 580 3614 3619 136 g (4.8 oz) 7 lbs, 16 oz
40W,6D 600 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 537 3658 3669 130 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 3719 114 g (4 oz) 8 lbs, 3 oz
41W,5D 263 3791 3739 120 g (4.2 oz) 8 lbs, 4 oz
41W,6D 220 3727 3750 112 g (4 oz) 8 lbs, 4 oz
42W,0D 203 3788 3771 117 g (4.1 oz) 8 lbs, 5 oz
42W,1D 117 3780 3793 124 g (4.4 oz) 8 lbs, 6 oz
42W,2D 68 3857 3797 109 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 3848 129 g (4.6 oz) 8 lbs, 8 oz
42W,5D 25 3877 3890 151 g (5.3 oz) 8 lbs, 9 oz
42W,6D 12 3898 3908 158 g (5.6 oz) 8 lbs, 10 oz
Total responses: 15186

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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