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 2756 6 lbs, 1 oz
35W,1D 26 2644 2754 6 lbs, 1 oz
35W,2D 24 2836 2744 6 lbs, 1 oz
35W,3D 33 2831 2746 6 lbs, 1 oz
35W,4D 21 2742 2779 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 2802 46 g (1.6 oz) 6 lbs, 3 oz
36W,1D 42 2624 2819 65 g (2.3 oz) 6 lbs, 3 oz
36W,2D 54 2862 2865 121 g (4.3 oz) 6 lbs, 5 oz
36W,3D 51 2935 2920 174 g (6.1 oz) 6 lbs, 7 oz
36W,4D 46 2889 2943 164 g (5.8 oz) 6 lbs, 8 oz
36W,5D 60 3008 2974 209 g (7.4 oz) 6 lbs, 9 oz
36W,6D 74 3110 3011 232 g (8.2 oz) 6 lbs, 10 oz
37W,0D 119 2979 3031 229 g (8.1 oz) 6 lbs, 11 oz
37W,1D 118 2968 3061 242 g (8.5 oz) 6 lbs, 12 oz
37W,2D 116 3110 3089 224 g (7.9 oz) 6 lbs, 13 oz
37W,3D 118 3085 3095 175 g (6.2 oz) 6 lbs, 13 oz
37W,4D 144 3145 3138 195 g (6.9 oz) 6 lbs, 15 oz
37W,5D 138 3202 3171 197 g (6.9 oz) 6 lbs, 16 oz
37W,6D 180 3130 3189 178 g (6.3 oz) 7 lbs, 0 oz
38W,0D 253 3221 3223 192 g (6.8 oz) 7 lbs, 2 oz
38W,1D 220 3223 3254 193 g (6.8 oz) 7 lbs, 3 oz
38W,2D 221 3245 3270 181 g (6.4 oz) 7 lbs, 3 oz
38W,3D 260 3324 3302 207 g (7.3 oz) 7 lbs, 4 oz
38W,4D 335 3341 3327 189 g (6.7 oz) 7 lbs, 5 oz
38W,5D 314 3318 3351 180 g (6.3 oz) 7 lbs, 6 oz
38W,6D 363 3376 3364 175 g (6.2 oz) 7 lbs, 7 oz
39W,0D 525 3368 3385 162 g (5.7 oz) 7 lbs, 7 oz
39W,1D 462 3406 3400 146 g (5.1 oz) 7 lbs, 8 oz
39W,2D 418 3379 3414 144 g (5.1 oz) 7 lbs, 8 oz
39W,3D 555 3456 3433 131 g (4.6 oz) 7 lbs, 9 oz
39W,4D 560 3448 3462 135 g (4.8 oz) 7 lbs, 10 oz
39W,5D 538 3439 3483 132 g (4.7 oz) 7 lbs, 11 oz
39W,6D 587 3511 3504 140 g (4.9 oz) 7 lbs, 12 oz
40W,0D 1095 3512 3522 137 g (4.8 oz) 7 lbs, 12 oz
40W,1D 700 3559 3539 139 g (4.9 oz) 7 lbs, 13 oz
40W,2D 634 3568 3560 146 g (5.1 oz) 7 lbs, 14 oz
40W,3D 595 3599 3576 143 g (5 oz) 7 lbs, 14 oz
40W,4D 568 3595 3605 143 g (5 oz) 7 lbs, 15 oz
40W,5D 582 3614 3619 136 g (4.8 oz) 7 lbs, 16 oz
40W,6D 602 3638 3639 135 g (4.8 oz) 8 lbs, 0 oz
41W,0D 659 3662 3654 132 g (4.7 oz) 8 lbs, 1 oz
41W,1D 541 3659 3669 130 g (4.6 oz) 8 lbs, 1 oz
41W,2D 439 3721 3688 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 434 3718 3702 126 g (4.4 oz) 8 lbs, 3 oz
41W,4D 328 3715 3720 115 g (4.1 oz) 8 lbs, 3 oz
41W,5D 263 3791 3740 121 g (4.3 oz) 8 lbs, 4 oz
41W,6D 220 3727 3749 110 g (3.9 oz) 8 lbs, 4 oz
42W,0D 204 3790 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 69 3848 3797 109 g (3.8 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 25 3877 3886 146 g (5.1 oz) 8 lbs, 9 oz
42W,6D 12 3898 3908 159 g (5.6 oz) 8 lbs, 10 oz
Total responses: 15278

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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