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 21 2723 2770 6 lbs, 2 oz
35W,1D 24 2660 2765 6 lbs, 2 oz
35W,2D 24 2836 2753 6 lbs, 1 oz
35W,3D 33 2831 2754 6 lbs, 1 oz
35W,4D 21 2742 2782 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 23 g (0.8 oz) 6 lbs, 3 oz
36W,1D 42 2624 2812 47 g (1.7 oz) 6 lbs, 3 oz
36W,2D 53 2857 2858 105 g (3.7 oz) 6 lbs, 5 oz
36W,3D 49 2900 2914 160 g (5.6 oz) 6 lbs, 7 oz
36W,4D 46 2889 2939 157 g (5.5 oz) 6 lbs, 8 oz
36W,5D 59 3007 2972 207 g (7.3 oz) 6 lbs, 9 oz
36W,6D 73 3114 3012 234 g (8.3 oz) 6 lbs, 10 oz
37W,0D 116 2983 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 113 3121 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 201 g (7.1 oz) 6 lbs, 15 oz
37W,5D 133 3206 3170 198 g (7 oz) 6 lbs, 16 oz
37W,6D 179 3132 3188 176 g (6.2 oz) 7 lbs, 0 oz
38W,0D 245 3223 3222 188 g (6.6 oz) 7 lbs, 2 oz
38W,1D 209 3213 3253 188 g (6.6 oz) 7 lbs, 3 oz
38W,2D 215 3246 3268 176 g (6.2 oz) 7 lbs, 3 oz
38W,3D 255 3323 3300 202 g (7.1 oz) 7 lbs, 4 oz
38W,4D 330 3335 3324 184 g (6.5 oz) 7 lbs, 5 oz
38W,5D 308 3316 3349 179 g (6.3 oz) 7 lbs, 6 oz
38W,6D 358 3376 3361 173 g (6.1 oz) 7 lbs, 7 oz
39W,0D 512 3362 3382 160 g (5.6 oz) 7 lbs, 7 oz
39W,1D 452 3407 3399 146 g (5.1 oz) 7 lbs, 8 oz
39W,2D 410 3377 3414 146 g (5.1 oz) 7 lbs, 8 oz
39W,3D 548 3455 3433 133 g (4.7 oz) 7 lbs, 9 oz
39W,4D 549 3450 3463 139 g (4.9 oz) 7 lbs, 10 oz
39W,5D 530 3441 3484 135 g (4.8 oz) 7 lbs, 11 oz
39W,6D 581 3512 3505 144 g (5.1 oz) 7 lbs, 12 oz
40W,0D 1079 3513 3523 141 g (5 oz) 7 lbs, 12 oz
40W,1D 688 3559 3540 141 g (5 oz) 7 lbs, 13 oz
40W,2D 625 3569 3560 146 g (5.1 oz) 7 lbs, 14 oz
40W,3D 587 3599 3576 143 g (5 oz) 7 lbs, 14 oz
40W,4D 561 3595 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 596 3638 3639 134 g (4.7 oz) 8 lbs, 0 oz
41W,0D 651 3662 3654 131 g (4.6 oz) 8 lbs, 1 oz
41W,1D 533 3659 3669 129 g (4.6 oz) 8 lbs, 1 oz
41W,2D 429 3726 3689 129 g (4.6 oz) 8 lbs, 2 oz
41W,3D 428 3720 3702 126 g (4.4 oz) 8 lbs, 3 oz
41W,4D 322 3715 3721 116 g (4.1 oz) 8 lbs, 3 oz
41W,5D 258 3793 3740 121 g (4.3 oz) 8 lbs, 4 oz
41W,6D 216 3722 3749 110 g (3.9 oz) 8 lbs, 4 oz
42W,0D 202 3788 3770 116 g (4.1 oz) 8 lbs, 5 oz
42W,1D 115 3775 3791 122 g (4.3 oz) 8 lbs, 6 oz
42W,2D 67 3847 3794 105 g (3.7 oz) 8 lbs, 6 oz
42W,3D 48 4032 3828 126 g (4.4 oz) 8 lbs, 7 oz
42W,4D 19 3891 3844 123 g (4.3 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: 15028

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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