Week 1 - Abnormalities
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 the trisomies, the most significant statistically being Trisomy 21 .
Failure of blastocyst development to form an embryoblast (inner cell mass) may also contribute to the later forming abnormality of Hydatidiform Mole ((hydatiform mole, hydatid mole, molar pregnancy, gestational trophoblastic disease).
Monozygotic twinning can also occur during this first week of development.
|Week 1 Links: stage 1 | stage 2 | stage 3 | menstrual cycle | fertilization | zygote | morula | blastocyst | Lecture - Fertilization | meiosis | mitosis | Lecture - Week 1 and 2 | menstrual cycle | oocyte | spermatozoa | twinning | Genetic risk maternal age | Trisomy 21 | Trisomy 18 | Trisomy 13 | hydatidiform mole | GA week 3|
Monozygotic 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.
Table based upon: Twinning. Hall JG. 
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.
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Cite this page: Hill, M.A. (2021, July 28) Embryology Week 1 - Abnormalities. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Week_1_-_Abnormalities
- © Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G