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.
| Date | # Births | Average (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 | 41 | 2621 | 2809 | 57 g (2 oz) | 6 lbs, 3 oz |
| 36W,2D | 50 | 2845 | 2856 | 113 g (4 oz) | 6 lbs, 5 oz |
| 36W,3D | 48 | 2893 | 2915 | 170 g (6 oz) | 6 lbs, 7 oz |
| 36W,4D | 43 | 2897 | 2940 | 166 g (5.9 oz) | 6 lbs, 8 oz |
| 36W,5D | 56 | 3015 | 2970 | 212 g (7.5 oz) | 6 lbs, 9 oz |
| 36W,6D | 73 | 3114 | 3011 | 236 g (8.3 oz) | 6 lbs, 10 oz |
| 37W,0D | 116 | 2983 | 3034 | 245 g (8.6 oz) | 6 lbs, 11 oz |
| 37W,1D | 108 | 2957 | 3064 | 255 g (9 oz) | 6 lbs, 12 oz |
| 37W,2D | 109 | 3122 | 3091 | 235 g (8.3 oz) | 6 lbs, 13 oz |
| 37W,3D | 117 | 3085 | 3095 | 180 g (6.3 oz) | 6 lbs, 13 oz |
| 37W,4D | 137 | 3146 | 3138 | 198 g (7 oz) | 6 lbs, 15 oz |
| 37W,5D | 132 | 3207 | 3170 | 200 g (7.1 oz) | 6 lbs, 16 oz |
| 37W,6D | 172 | 3121 | 3187 | 176 g (6.2 oz) | 7 lbs, 0 oz |
| 38W,0D | 234 | 3225 | 3221 | 187 g (6.6 oz) | 7 lbs, 2 oz |
| 38W,1D | 206 | 3212 | 3252 | 188 g (6.6 oz) | 7 lbs, 3 oz |
| 38W,2D | 212 | 3244 | 3266 | 175 g (6.2 oz) | 7 lbs, 3 oz |
| 38W,3D | 251 | 3322 | 3301 | 206 g (7.3 oz) | 7 lbs, 4 oz |
| 38W,4D | 326 | 3334 | 3322 | 184 g (6.5 oz) | 7 lbs, 5 oz |
| 38W,5D | 303 | 3315 | 3347 | 177 g (6.2 oz) | 7 lbs, 6 oz |
| 38W,6D | 350 | 3380 | 3360 | 173 g (6.1 oz) | 7 lbs, 7 oz |
| 39W,0D | 506 | 3357 | 3380 | 159 g (5.6 oz) | 7 lbs, 7 oz |
| 39W,1D | 446 | 3405 | 3397 | 145 g (5.1 oz) | 7 lbs, 8 oz |
| 39W,2D | 406 | 3376 | 3412 | 146 g (5.1 oz) | 7 lbs, 8 oz |
| 39W,3D | 537 | 3449 | 3431 | 130 g (4.6 oz) | 7 lbs, 9 oz |
| 39W,4D | 542 | 3450 | 3462 | 140 g (4.9 oz) | 7 lbs, 10 oz |
| 39W,5D | 523 | 3441 | 3483 | 136 g (4.8 oz) | 7 lbs, 11 oz |
| 39W,6D | 569 | 3515 | 3504 | 144 g (5.1 oz) | 7 lbs, 12 oz |
| 40W,0D | 1052 | 3515 | 3522 | 142 g (5 oz) | 7 lbs, 12 oz |
| 40W,1D | 674 | 3555 | 3540 | 143 g (5 oz) | 7 lbs, 13 oz |
| 40W,2D | 614 | 3568 | 3560 | 148 g (5.2 oz) | 7 lbs, 14 oz |
| 40W,3D | 568 | 3598 | 3575 | 144 g (5.1 oz) | 7 lbs, 14 oz |
| 40W,4D | 552 | 3596 | 3603 | 141 g (5 oz) | 7 lbs, 15 oz |
| 40W,5D | 572 | 3614 | 3618 | 135 g (4.8 oz) | 7 lbs, 16 oz |
| 40W,6D | 589 | 3633 | 3638 | 134 g (4.7 oz) | 8 lbs, 0 oz |
| 41W,0D | 638 | 3662 | 3653 | 131 g (4.6 oz) | 8 lbs, 1 oz |
| 41W,1D | 523 | 3660 | 3668 | 128 g (4.5 oz) | 8 lbs, 1 oz |
| 41W,2D | 419 | 3725 | 3688 | 128 g (4.5 oz) | 8 lbs, 2 oz |
| 41W,3D | 416 | 3721 | 3702 | 127 g (4.5 oz) | 8 lbs, 3 oz |
| 41W,4D | 315 | 3714 | 3720 | 117 g (4.1 oz) | 8 lbs, 3 oz |
| 41W,5D | 250 | 3792 | 3740 | 122 g (4.3 oz) | 8 lbs, 4 oz |
| 41W,6D | 214 | 3719 | 3748 | 110 g (3.9 oz) | 8 lbs, 4 oz |
| 42W,0D | 199 | 3790 | 3769 | 116 g (4.1 oz) | 8 lbs, 5 oz |
| 42W,1D | 114 | 3773 | 3791 | 123 g (4.3 oz) | 8 lbs, 6 oz |
| 42W,2D | 67 | 3847 | 3794 | 106 g (3.7 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 | 23 | 3909 | 3890 | 150 g (5.3 oz) | 8 lbs, 9 oz |
| 42W,6D | 12 | 3898 | 3914 | 166 g (5.9 oz) | 8 lbs, 10 oz |