Week 1
Introduction
Key Events of Human Development during the first week (week 1) following fertilization or Clinical week 3 (LMP).
Links: 2009 Lecture - Fertilization | 2009 Lecture - Week 1 and 2 | Oocyte | Spermatozoa | Fertilization
Human-oocyte to blastocyst
| Event | ||
| Secretory PhaseStage 1 |  Fertilization, Secretory Phase
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| Stage 2 |   Morula, Blastula
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| Stage 3 |  Blastocyst Hatching (zona pellucida lost)
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Blastocyst (free floating) | ||
| Stage 4 | Adplantation | |
| Stage 5 | File:2ndwk.jpg |
Week 1 Movies
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| Ovulation | Fertilization | Pronuclear Fusion | Week 1 | Ovulation in the rabbit | Fertilization in the mouse |
Week 1 Abnormalities
Additional Information
Dizygotic Twinning
Dizygotic twins (fraternal, non-identical) arise from separate fertilization events involving two separate oocyte (egg, ova) and spermatozoa (sperm).
Monoygotic Twinning
Monoygotic twins (identical) produced from a single fertilization event (one fertilised egg and a single spermatazoa, form a single zygote), these twins therefore share the same genetic makeup. Occurs in approximately 3-5 per 1000 pregnancies, more commonly with aged mothers. The later the twinning event, the less common are initially separate placental membranes and finally resulting in conjoined twins.
| Week | Week 1 | Week 2 | |||||||||||||
| Day | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
| Cell Number | 1 | 1 | 2 | 16 | 32 | 128 | bilaminar | ||||||||
| Event | Ovulation | fertilization | First cell division | Morula | Early blastocyst | Late blastocyst
Hatching |
Implantation starts | X inactivation | |||||||
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| Monoygotic
Twin Type |
Diamniotic
Dichorionic |
Diamniotic
Monochorionic |
Monoamniotic
Monochorionic |
Conjoined | |||||||||||
Table based upon: Twinning. Hall JG. Lancet. 2003 Aug 30;362(9385):735-43. Review. PMID: 12957099
Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9
- Carnegie Stages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | About Stages | Timeline
Glossary Links
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2024, May 4) Embryology Week 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Week_1
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
