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 2778 6 lbs, 2 oz
36W,0D 51 2856 2793 34 g (1.2 oz) 6 lbs, 3 oz
36W,1D 42 2624 2812 56 g (2 oz) 6 lbs, 3 oz
36W,2D 53 2857 2858 112 g (4 oz) 6 lbs, 5 oz
36W,3D 49 2900 2914 166 g (5.9 oz) 6 lbs, 7 oz
36W,4D 46 2889 2940 160 g (5.6 oz) 6 lbs, 8 oz
36W,5D 59 3007 2973 208 g (7.3 oz) 6 lbs, 9 oz
36W,6D 73 3114 3013 235 g (8.3 oz) 6 lbs, 10 oz
37W,0D 117 2986 3034 241 g (8.5 oz) 6 lbs, 11 oz
37W,1D 114 2971 3065 253 g (8.9 oz) 6 lbs, 12 oz
37W,2D 114 3118 3092 234 g (8.3 oz) 6 lbs, 13 oz
37W,3D 118 3085 3098 184 g (6.5 oz) 6 lbs, 13 oz
37W,4D 143 3143 3140 200 g (7.1 oz) 6 lbs, 15 oz
37W,5D 136 3202 3169 196 g (6.9 oz) 6 lbs, 16 oz
37W,6D 179 3132 3187 174 g (6.1 oz) 7 lbs, 0 oz
38W,0D 248 3222 3221 187 g (6.6 oz) 7 lbs, 2 oz
38W,1D 210 3209 3253 188 g (6.6 oz) 7 lbs, 3 oz
38W,2D 217 3247 3268 176 g (6.2 oz) 7 lbs, 3 oz
38W,3D 255 3323 3301 203 g (7.2 oz) 7 lbs, 4 oz
38W,4D 332 3339 3324 184 g (6.5 oz) 7 lbs, 5 oz
38W,5D 310 3316 3350 181 g (6.4 oz) 7 lbs, 6 oz
38W,6D 358 3376 3362 175 g (6.2 oz) 7 lbs, 7 oz
39W,0D 517 3363 3383 162 g (5.7 oz) 7 lbs, 7 oz
39W,1D 453 3408 3400 147 g (5.2 oz) 7 lbs, 8 oz
39W,2D 412 3377 3414 146 g (5.1 oz) 7 lbs, 8 oz
39W,3D 551 3457 3434 133 g (4.7 oz) 7 lbs, 9 oz
39W,4D 551 3450 3463 139 g (4.9 oz) 7 lbs, 10 oz
39W,5D 532 3441 3484 134 g (4.7 oz) 7 lbs, 11 oz
39W,6D 583 3513 3505 143 g (5 oz) 7 lbs, 12 oz
40W,0D 1081 3514 3523 140 g (4.9 oz) 7 lbs, 12 oz
40W,1D 692 3560 3540 140 g (4.9 oz) 7 lbs, 13 oz
40W,2D 627 3568 3560 146 g (5.1 oz) 7 lbs, 14 oz
40W,3D 588 3598 3576 142 g (5 oz) 7 lbs, 14 oz
40W,4D 563 3594 3605 142 g (5 oz) 7 lbs, 15 oz
40W,5D 578 3615 3619 135 g (4.8 oz) 7 lbs, 16 oz
40W,6D 597 3637 3638 133 g (4.7 oz) 8 lbs, 0 oz
41W,0D 653 3662 3654 131 g (4.6 oz) 8 lbs, 1 oz
41W,1D 534 3658 3669 129 g (4.6 oz) 8 lbs, 1 oz
41W,2D 433 3722 3688 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 432 3719 3702 126 g (4.4 oz) 8 lbs, 3 oz
41W,4D 322 3715 3720 115 g (4.1 oz) 8 lbs, 3 oz
41W,5D 260 3793 3741 122 g (4.3 oz) 8 lbs, 4 oz
41W,6D 218 3728 3750 112 g (4 oz) 8 lbs, 4 oz
42W,0D 202 3788 3772 118 g (4.2 oz) 8 lbs, 5 oz
42W,1D 116 3783 3794 125 g (4.4 oz) 8 lbs, 6 oz
42W,2D 68 3857 3798 110 g (3.9 oz) 8 lbs, 6 oz
42W,3D 48 4032 3832 130 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 149 g (5.3 oz) 8 lbs, 9 oz
42W,6D 12 3898 3908 158 g (5.6 oz) 8 lbs, 10 oz
Total responses: 15092

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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