| Days | Size in millimetres | ||||||
| LMP | PF | 50th | 5th | 95th | %diff. 50th to 5th | % diff. 50th to 95th | % diff. 95th to 5th |
| 40 | 26 | 2.4 | 1.1 | 4.1 | 118% | 41% | 273% |
| 41 | 27 | 2.9 | 1.4 | 4.8 | 107% | 40% | 243% |
| 42 | 28 | 3.4 | 1.9 | 5.5 | 79% | 38% | 189% |
| 43 | 29 | 4.1 | 2.3 | 6.3 | 78% | 35% | 174% |
| 44 | 30 | 4.7 | 2.8 | 7.1 | 68% | 34% | 154% |
| 45 | 31 | 5.4 | 3.4 | 7.9 | 59% | 32% | 132% |
| 46 | 32 | 6.1 | 3.9 | 8.8 | 56% | 31% | 126% |
| 47 | 33 | 6.9 | 4.5 | 9.7 | 53% | 29% | 116% |
| 48 | 34 | 7.7 | 5.2 | 10.6 | 48% | 27% | 104% |
| 49 | 35 | 8.5 | 5.9 | 11.6 | 44% | 27% | 97% |
| 50 | 36 | 9.4 | 6.6 | 12.6 | 42% | 25% | 91% |
| 51 | 37 | 10.2 | 7.3 | 13.6 | 40% | 25% | 86% |
| 52 | 38 | 11.2 | 8.1 | 14.7 | 38% | 24% | 81% |
| 53 | 39 | 12.1 | 8.9 | 15.7 | 36% | 23% | 76% |
| 54 | 40 | 13.0 | 9.7 | 16.8 | 34% | 23% | 73% |
| 55 | 41 | 14.0 | 10.6 | 17.9 | 32% | 22% | 69% |
| 56 | 42 | 15.0 | 11.4 | 19.1 | 32% | 21% | 68% |
| 57 | 43 | 16.0 | 12.3 | 20.2 | 30% | 21% | 64% |
| 58 | 44 | 17.1 | 13.2 | 21.4 | 30% | 20% | 62% |
| 59 | 45 | 18.1 | 14.2 | 22.5 | 27% | 20% | 58% |
| 60 | 46 | 19.1 | 15.1 | 23.7 | 26% | 19% | 57% |
| 61 | 47 | 20.2 | 16.0 | 24.9 | 26% | 19% | 56% |
| 62 | 48 | 21.3 | 17.0 | 26.1 | 25% | 18% | 54% |
| 63 | 49 | 22.4 | 18.0 | 27.3 | 24% | 18% | 52% |
| 64 | 50 | 23.5 | 18.9 | 28.5 | 24% | 18% | 51% |
| 65 | 51 | 24.6 | 19.9 | 29.7 | 24% | 17% | 49% |
| 66 | 52 | 25.7 | 20.9 | 30.9 | 23% | 17% | 48% |
| 67 | 53 | 26.8 | 21.9 | 32.1 | 22% | 17% | 47% |
| 68 | 54 | 27.9 | 22.9 | 33.3 | 22% | 16% | 45% |
| 69 | 55 | 29.0 | 23.9 | 34.5 | 21% | 16% | 44% |
| 70 | 56 | 30.1 | 24.9 | 35.7 | 21% | 16% | 43% |
| 71 | 57 | 31.2 | 25.9 | 36.9 | 20% | 15% | 42% |
| 72 | 58 | 32.3 | 26.9 | 38.1 | 20% | 15% | 42% |
| 73 | 59 | 33.3 | 27.9 | 39.3 | 19% | 15% | 41% |
| 74 | 60 | 34.4 | 28.9 | 40.4 | 19% | 15% | 40% |
| 75 | 61 | 35.5 | 29.9 | 41.6 | 19% | 15% | 39% |
This is a chart showing the norms for Crown-Rump Length measurements for embryos and first trimester fetuses in millimetres. The source for this information is below.
LMP= Days sinnce Last Menstrual Period. That's how OB-GYN's and other medical professionals calculate the age of a pregnancy.
PF= Post-fertilization. That's how embryologists calculate the age of an embryo. If I wanted to be really exact, some of them use post-ovulatory age, which is slightly different than post-fertilization, but for the purposes of the exercise, it's not a dig deal.
I marked down the ages according to the 50th percentile, the 5th percentile and the 95th percentile, so that you can know what's a small embryo for that age, and what's a large one.
I took the liberty to calculate the difference between the different percentiles to underscore that there can be significant varations in measurements in the early weeks.
To accurately estimate the age of an embryo, you might also want to consult the 23 Carnegie Stages of embryonic development.
Citation:
Normal Ranges of Embryonic Length, Embryonic
Heart Rate, Gestational Sac Diameter and Yolk Sac
Diameter at 6–10 Weeks
George I. Papaioannou Argyro Syngelaki Leona C.Y. Poon Jackie A. Ross
Kypros H. Nicolaides
Harris Birthright Research Centre for Fetal Medicine and Early Pregnancy Unit, King’s College Hospital, London , UK
Fetal Diagn Ther 2010;28:207–219
DOI: 10.1159/000319589
