Lecture 3: Conception and Pregnancy Flashcards
how long is pregnancy and the menstrual cycle? how is this used to calculate the due date?
3 trimesters
- 1st 10-11 weeks is embryogenesis, then it is fetal development
Mentrual cycle = 4 weeks
- 1st week is that start of period
- ovulation is in the middle of the 4 weeks.
Due date:
- first day of last period + 280 days (=~40 weeks)
- for the first 2 weeks of this you are not even pregnant - but we start at start of last period as it is hard to tell when ovulation happens
what is required for fertilisation?
gametes - haploid (one set of chromosomes), reproduction cells
meiosis - cell division which reduces the number of chromosomes by half
what are the steps of fertilisation and complete oocyte meiosis? where does it take place?
- Sperm enters recently ovulated oocyte. “acrosome reaction”. Sperm is taken up into center of oocyte and reaction happens in the outer layer of the oocyte (zona pellucida). Stops any other sperm from getting into the oocyte. When sperm hits zona pellucida, it starts to activate the final step of meiosis for the oocyte.
- The oocyte will undergo an asymmetrical cleavage. This means it will divide into 2 gametes with 23 chromosomes. Only one of them is viable, while the other one becomes a polar body and is not involved in the generation of a new individual. The sperm is sucked into the inside of the oocyte.
- 2 pronuclei (zygotes) starts undergoing mitotic division with mixed genetic material from 2 gametes
- this happens in the far end of the uterine horn
- sperm undergoes fertilisation and a zygote forms which undergoes cell division for the next 5 days
- then a cavity forms - called a bastocyst
what is the anatomy of a blastocyst?
- the inner cell mass becomes the embryo and amnion
- the trophoblast cells layer becomes the placental tissues (chorion)
the placenta is made from the placental tissues, but also from the mother’s uterus.
when/where does implantation occur? what comes after implantation?
After 5 days of the blastocyst forming, it starts to connect and invade to the wall of the uterus (decidua) in the process of implantation
- embryo implantation (blastocyst-uterine interaction) initiates stable changes to the reproductive endocrine system (from cyclic patterns) to enable pregnany, parturition and lactation
- When blastocyst implants onto uterine wall, it releases a signal to prepare the body for pregnancy.
Normally, after 10 days it just goes back to normal. So releases the built up lining on the uterus. this signal is “chorianic gonadotrophin”
how does chrorionic gonadotrophin ac as a signal?
trophoblast tissue attaches to and invades the endrometrium
- after invasion: secretes chrionoic gonadotrophin which prevents luteolysis (i.e. corpus luteum does not degenerate and continues to produce progesterone)
- then is establishes placental tissues
what are the simple steps of pregnancy?
zygote -> blastocyst -> embryo (structure formation) -> fetus (growth and development - after 10-11 weeks)
what are the functions of the placenta?
exchange from mother to fetus:
- O2, nutrients e.g. glucose, hormones, maternal antibodies
- drugs and alcohol get exchanged too!
exchange from fetus to mother:
- CO2, waste products
hormone production:
- human chorionic gonadotrophin (hcG) - maintains progesterone production, prevents menstruation
- estrogen - stimulate uterine growth
- placental lactogen - increases availability of maternal glucose for promoting fetal growth, promotes breast development
what happens around 3 weeks gestation on the fetal side and maternal side?
- placental villi begin to form around week 3 of gestation
- *fetal side:**
- chorion form quickly and form chorionic villi which invade the uterine decidua/wall (decidua is part of the endometrium that undergoes modification to prepare for pregnancy, induced by progesteron)
- trophoblast cells forming outer lay of blastocyst
- cytotrophoblasts form villi, which are considered as stem cells for syncytiotrophoblasts
syncytiotrophoblasts fuse to the outer trophoblasts forming a specialised layer of epithelial cells that are in direct contact with maternal blood
maternal side:
- spiral arteries that supply the endometrium become less convoluted and increase their diameter (intraplacental lacunua)
-
what happens at around 8-12 weeks?
from 8-12 weeks the chorion is well defined and the amnion and amniotic cavity surrounds the fetus
- As the blastocyst grows and becomes an embryo and a fetus, it pushes into to the uterus cavity and all of the villi on the outer side disappear and only have them on one side of the uterus wall which is where the placenta will form.
what are the features and dunction of the amnion and amniotic fluid?
- suspending the embryo
- fluid is produced partly by the amniotic cells but the majority is from the maternal blood
- replaced every 3 hours
- from the 5th month the fetus swallows 400ml/day
- acts as a shock absorber
- allows movement
- prevents desiccation
- prevents adhesions between the fetus and the surrounding membranes
what are some early developmental failures of the human conceptus?
~1 in 4 pregnancies end in miscarriage
- triploidy
- trisomy
what is triploidy?
presence if ab additional set of chromosomes
- it is the failure of oocyte to complete 2nd meiotic division so polar body not formed
- more than one sperm fuses with oocyte
- less than 0.01% of triploid embryos survive birth
what is trisomy?
presence of an additional copy of one chromosome
Trisomy 21: down syndrome (1:1000 live births in NZ)
Trisomy 18: Edward Syndrome (1:2500 live births in New Zealand)
Trisomy 13: Patau syndrome (1:10000 live births in NZ)
risk factors: maternal age
what are the optional screening tests for genetic disorders?
before 14 weeks and between
Blood test at 9-10 weeks
- beta-human chorionic gonadotrophin = increased risk
PAPP-A (pregnancy associated plasma protein A = increased risk
nuchal translucency scan (ultrasound scan) around 12 weeks
optional blood test at 14-20 weeks
what is a nuchal translucency (NT) scan?
fetuses with chromosome, cardiac and other abnormalities have increased NT thickness due to fluid accumulation
