Returning to the topic of human embryology, here is a human fetal timeline for the first 16 weeks post fertilisation (obstetricians count from the LMP = last menstrual period, which adds about 2 weeks). It is a little disturbing quite how much scientific misinformation is being circulated in regard to the topic. False information is not conducive to honest debate, and is highly corrosive of the trust people have in professionals such as scientists (it’s also unethical on both religious and Kantian grounds). In particular, contrary to what some have suggested:
- the fetal or embryonic heart begins beating at about 21 days after conception, as any textbook will point out
- as the table shows, after a few weeks, the fetus or embryo is not “too small to see with the naked eye”
Fetal length data in the table is mostly from here. Except where indicated, linked images are subsequent to miscarriage or to surgery to resolve ectopic pregnancy, so may be distressing to some readers.
| Week post fertilisation | Week post LMP | Fetal length | Image |
|---|---|---|---|
| 1 | 3 | 0.01 cm / 0.005 inches | 8–cell image |
| 2 | 4 | 0.02 cm / 0.008 inches | – |
| 3 | 5 | 0.1 cm / 0.04 inches | heart begins beating at 21 days |
| 4 | 6 | 0.5 cm / 0.2 inches | image on flickr |
| 5 | 7 | 1 cm / 0.4 inches | image on wikimedia |
| 6 | 8 | 1.6 cm / 0.6 inches | image on flickr |
| 7 | 9 | 2.3 cm / 0.9 inches | image on flickr |
| 8 | 10 | 3.2 cm / 1.3 inches | – |
| 9 | 11 | 4.1 cm / 1.6 inches | – |
| 10 | 12 | 5.4 cm / 2.1 inches | – |
| 11 | 13 | 6.7 cm / 2.6 inches | ultrasound image |
| 12 | 14 | 14.7 cm / 5.8 inches | – |
| 13 | 15 | 16.7 cm / 6.6 inches | – |
| 14 | 16 | 18.6 cm / 7.3 inches | – |
| 15 | 17 | 20.4 cm / 8 inches | ultrasound image |
| 16 | 18 | 22 cm / 8.7 inches | – |
Below (from here) is a chart of heart development:
Scientific Gems 