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 42 2624 2808 56 g (2 oz) 6 lbs, 3 oz
36W,2D 50 2845 2857 114 g (4 oz) 6 lbs, 5 oz
36W,3D 48 2893 2915 170 g (6 oz) 6 lbs, 7 oz
36W,4D 45 2890 2939 165 g (5.8 oz) 6 lbs, 8 oz
36W,5D 57 3020 2971 213 g (7.5 oz) 6 lbs, 9 oz
36W,6D 73 3114 3012 237 g (8.4 oz) 6 lbs, 10 oz
37W,0D 116 2983 3034 245 g (8.6 oz) 6 lbs, 11 oz
37W,1D 111 2961 3065 257 g (9.1 oz) 6 lbs, 12 oz
37W,2D 110 3122 3091 234 g (8.3 oz) 6 lbs, 13 oz
37W,3D 118 3085 3096 181 g (6.4 oz) 6 lbs, 13 oz
37W,4D 139 3146 3138 199 g (7 oz) 6 lbs, 15 oz
37W,5D 132 3207 3170 199 g (7 oz) 6 lbs, 16 oz
37W,6D 174 3126 3187 175 g (6.2 oz) 7 lbs, 0 oz
38W,0D 237 3223 3221 187 g (6.6 oz) 7 lbs, 2 oz
38W,1D 206 3212 3252 187 g (6.6 oz) 7 lbs, 3 oz
38W,2D 212 3244 3267 176 g (6.2 oz) 7 lbs, 3 oz
38W,3D 251 3322 3301 205 g (7.2 oz) 7 lbs, 4 oz
38W,4D 328 3337 3323 185 g (6.5 oz) 7 lbs, 5 oz
38W,5D 304 3318 3348 178 g (6.3 oz) 7 lbs, 6 oz
38W,6D 351 3379 3361 174 g (6.1 oz) 7 lbs, 7 oz
39W,0D 508 3358 3382 161 g (5.7 oz) 7 lbs, 7 oz
39W,1D 447 3405 3399 147 g (5.2 oz) 7 lbs, 8 oz
39W,2D 408 3376 3413 146 g (5.1 oz) 7 lbs, 8 oz
39W,3D 541 3453 3432 131 g (4.6 oz) 7 lbs, 9 oz
39W,4D 546 3452 3463 140 g (4.9 oz) 7 lbs, 10 oz
39W,5D 524 3440 3484 136 g (4.8 oz) 7 lbs, 11 oz
39W,6D 572 3515 3505 144 g (5.1 oz) 7 lbs, 12 oz
40W,0D 1059 3514 3523 141 g (5 oz) 7 lbs, 12 oz
40W,1D 678 3557 3540 141 g (5 oz) 7 lbs, 13 oz
40W,2D 618 3569 3560 147 g (5.2 oz) 7 lbs, 14 oz
40W,3D 575 3597 3575 143 g (5 oz) 7 lbs, 14 oz
40W,4D 554 3596 3604 141 g (5 oz) 7 lbs, 15 oz
40W,5D 573 3614 3618 134 g (4.7 oz) 7 lbs, 16 oz
40W,6D 589 3633 3638 133 g (4.7 oz) 8 lbs, 0 oz
41W,0D 643 3663 3654 131 g (4.6 oz) 8 lbs, 1 oz
41W,1D 525 3658 3668 128 g (4.5 oz) 8 lbs, 1 oz
41W,2D 425 3724 3688 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 420 3721 3702 127 g (4.5 oz) 8 lbs, 3 oz
41W,4D 316 3716 3720 116 g (4.1 oz) 8 lbs, 3 oz
41W,5D 251 3791 3740 122 g (4.3 oz) 8 lbs, 4 oz
41W,6D 215 3721 3749 111 g (3.9 oz) 8 lbs, 4 oz
42W,0D 199 3790 3770 116 g (4.1 oz) 8 lbs, 5 oz
42W,1D 115 3775 3791 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 3831 129 g (4.6 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 165 g (5.8 oz) 8 lbs, 10 oz
Total responses: 14818

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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