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 2758 6 lbs, 1 oz
35W,1D 23 2652 2755 6 lbs, 1 oz
35W,2D 21 2780 2741 6 lbs, 1 oz
35W,3D 33 2831 2743 6 lbs, 1 oz
35W,4D 21 2742 2772 6 lbs, 2 oz
35W,5D 32 2688 2757 6 lbs, 1 oz
35W,6D 22 2757 2773 6 lbs, 2 oz
36W,0D 50 2859 2787 29 g (1 oz) 6 lbs, 2 oz
36W,1D 41 2621 2807 52 g (1.8 oz) 6 lbs, 3 oz
36W,2D 50 2845 2857 116 g (4.1 oz) 6 lbs, 5 oz
36W,3D 48 2893 2915 172 g (6.1 oz) 6 lbs, 7 oz
36W,4D 43 2897 2941 169 g (6 oz) 6 lbs, 8 oz
36W,5D 56 3015 2971 214 g (7.5 oz) 6 lbs, 9 oz
36W,6D 72 3119 3011 238 g (8.4 oz) 6 lbs, 10 oz
37W,0D 115 2986 3034 247 g (8.7 oz) 6 lbs, 11 oz
37W,1D 107 2956 3065 258 g (9.1 oz) 6 lbs, 12 oz
37W,2D 107 3117 3093 236 g (8.3 oz) 6 lbs, 13 oz
37W,3D 117 3085 3096 181 g (6.4 oz) 6 lbs, 13 oz
37W,4D 136 3149 3137 196 g (6.9 oz) 6 lbs, 15 oz
37W,5D 130 3213 3169 198 g (7 oz) 6 lbs, 16 oz
37W,6D 171 3119 3186 175 g (6.2 oz) 7 lbs, 0 oz
38W,0D 226 3221 3221 187 g (6.6 oz) 7 lbs, 2 oz
38W,1D 206 3212 3251 186 g (6.6 oz) 7 lbs, 3 oz
38W,2D 212 3244 3265 172 g (6.1 oz) 7 lbs, 3 oz
38W,3D 250 3324 3300 204 g (7.2 oz) 7 lbs, 4 oz
38W,4D 322 3331 3322 185 g (6.5 oz) 7 lbs, 5 oz
38W,5D 299 3314 3347 178 g (6.3 oz) 7 lbs, 6 oz
38W,6D 349 3381 3360 174 g (6.1 oz) 7 lbs, 7 oz
39W,0D 503 3357 3380 159 g (5.6 oz) 7 lbs, 7 oz
39W,1D 440 3405 3397 146 g (5.1 oz) 7 lbs, 8 oz
39W,2D 405 3376 3412 147 g (5.2 oz) 7 lbs, 8 oz
39W,3D 532 3450 3431 131 g (4.6 oz) 7 lbs, 9 oz
39W,4D 539 3449 3462 140 g (4.9 oz) 7 lbs, 10 oz
39W,5D 519 3441 3483 136 g (4.8 oz) 7 lbs, 11 oz
39W,6D 565 3514 3504 144 g (5.1 oz) 7 lbs, 12 oz
40W,0D 1043 3516 3522 142 g (5 oz) 7 lbs, 12 oz
40W,1D 668 3556 3539 142 g (5 oz) 7 lbs, 13 oz
40W,2D 609 3568 3560 148 g (5.2 oz) 7 lbs, 14 oz
40W,3D 565 3598 3575 144 g (5.1 oz) 7 lbs, 14 oz
40W,4D 546 3592 3603 141 g (5 oz) 7 lbs, 15 oz
40W,5D 567 3616 3618 135 g (4.8 oz) 7 lbs, 16 oz
40W,6D 587 3632 3637 133 g (4.7 oz) 8 lbs, 0 oz
41W,0D 629 3662 3653 131 g (4.6 oz) 8 lbs, 1 oz
41W,1D 518 3659 3669 130 g (4.6 oz) 8 lbs, 1 oz
41W,2D 413 3725 3688 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 414 3723 3702 127 g (4.5 oz) 8 lbs, 3 oz
41W,4D 312 3715 3720 117 g (4.1 oz) 8 lbs, 3 oz
41W,5D 248 3792 3741 123 g (4.3 oz) 8 lbs, 4 oz
41W,6D 213 3717 3750 113 g (4 oz) 8 lbs, 4 oz
42W,0D 198 3787 3770 117 g (4.1 oz) 8 lbs, 5 oz
42W,1D 111 3786 3792 123 g (4.3 oz) 8 lbs, 6 oz
42W,2D 67 3847 3795 107 g (3.8 oz) 8 lbs, 6 oz
42W,3D 48 4032 3833 131 g (4.6 oz) 8 lbs, 7 oz
42W,4D 19 3891 3851 131 g (4.6 oz) 8 lbs, 8 oz
42W,5D 23 3909 3890 149 g (5.3 oz) 8 lbs, 9 oz
42W,6D 12 3898 3914 164 g (5.8 oz) 8 lbs, 10 oz
Total responses: 14610

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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