| Unit 1: The First Week | |
| Day 0 |
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 Fertilization |
| Day 5 |
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Blastocyst |
| Day 6 |
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Implantation begins |
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Implantation begins - blastocyst attaches to uterine wall |
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| Unit 2: 1 to 2 Weeks | |
| 1 week, 6 days |
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Primitive streak |
| Unit 3: 2 to 3 Weeks | |
| 2 weeks, 3 days |
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 Early thyroid |
| Unit 4: 3 to 4 Weeks | |
| 3 weeks, 1 day - 4 weeks |
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 Folding of the embryo |
| 4 weeks |
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 Heart rate 113 beats/min |
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 Upper & lower limb buds |
| Unit 5: 4 to 5 Weeks | |
| 5 weeks |
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 Head is 1/3 of entire embryo |
| Unit 6: 5 to 6 Weeks | |
| 6 weeks |
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 Face withdraws to light touch around mouth |
| Unit 7: 6 to 7 Weeks | |
| 7 weeks |
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Heart rate peaks 170 beats/minute |
| Unit 8: 7 to 8 Weeks | |
| 7 ½ weeks |
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Knee joints |
| 8 weeks |
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 Breathing motion |
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 Urination |
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 Brain highly developed |
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| Unit 9: 8 to 9 Weeks | |
| 9 weeks |
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Thyroid gland weighs 2 grams |
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 Sighing |
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Sucking |
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Swallowing |
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Thumb sucking |
| Unit 10: 9 to 10 Weeks | |
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| Unit 11: 10 to 11 Weeks | |
| 11 weeks |
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 Nose & lips completely formed |
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Thyroid gland weighs 12 grams |
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| Unit 12: 11 to 12 Weeks | |
| 12 weeks |
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Taste buds throughout mouth |
| Unit 13: 3 to 4 Months | |
| 14 weeks |
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Jaw motion more frequent in females |
| 15 weeks |
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 Fat deposits in abdomen, back, kidneys, and shoulders |
| 16 weeks |
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Fat deposits upper and lower limbs |
| Unit 14: 4 to 5 Months | |
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| Unit 15: 5 to 6 Months | |
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| Unit 16: 6 to 7 Months | |
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| Unit 17: 7 to 8 Months | |
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| Unit 18: 8 to 9 Months | |
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| Unit 19: 9 Months to Birth | |
| 38 weeks |
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Birth |
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There are 262,144 user-selectable configurations of this timeline. |
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Heart beats 54 million times before birth |