Week 1 - Abnormalities

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Introduction

Week 1 and 2 development

Many major genetic abnormalities occurring within, either the oocyte or spermatozoa, or as a result of their combination in the zygote, generally fail to develop through the blastocyst stage to hatch and implant in the second week of development. These can occur without any recognition of a pregnancy occurring.

There are only a few major chromosomal abnormalities that appear to escape this early biological "genetic screening" process and include Trisomy 21.


Key Events of Human Development during the first week (week 1) following fertilization or clinical gestational age week 3, base on last menstrual period.

The first week of human development begins with fertilization of the egg by sperm forming the zygote, followed by early cell division forming the blastocyst. These notes also cover events before fertilization formation of both the egg and sperm, gametogenesis.

Initially, there is a halving of chromosomal content in the gametes, which is restored by fertilization, allowing genetic recombination to occur. This is then followed by a series of cell divisions without cytoplasmic growth. During this first week the egg, then zygote, then the blastula is moving along the uterine horn into the uterus for implantation in the uterine wall.

Implantation also begins in this first week, but will be covered in Week 2 notes, as the implantation process is completed by the end of the second week.


Week 1 Links: Lecture - Fertilization | Meiosis | Mitosis | Lecture - Week 1 and 2 | Menstrual Cycle | Oocyte | Spermatozoa | Fertilization | Zygote | Morula | Blastocyst
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