Ovulation and Conception

"Stargazer Lily" by Derek Ramsey (Ram-Man) , used under CC BY SA 2.0/Cropped from original

Ovulation

The hypothalamic-pituitary-ovarian axis initiates ovulation. The hypothalamus will secrete gonadotropin-releasing hormone (GnRH) to stimulate the anterior pituitary to secrete luteinizing hormone (LH) and follicle stimulating hormone (FSH). These stimulate the ovaries, which bring about ovulation. If pregnancy occurs, the corpus luteal cyst (where the egg was released on the ovary) will produce progesterone to continue the pregnancy. If no pregnancy occurs, there will be a drop in estrogen and progesterone hormones, and it will signal back to the hypothalamus to begin the cycle again.

The Menstrual Cycle | Geeky MedicsAn overview of the menstrual cycle including the HPG axis, follicular phase, luteal phase and the uterine cycle.

GeekyMedics, (2011, May 18). The Menstrual Cycle: https://geekymedics.com/the-menstrual-cycle/

"Hormone Levels During the Menstrual Cycle" by Drguttorm., used under CC BY-SA 3.0/Cropped from original

"Axis Hypothalamicus Pituitarius Gonadalis" by Artoria2e5, used under CC BY SA 4.0/Cropped from original

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Conception

Conception requires correct timing between the release of the mature ovum (egg) at ovulation and ejaculation of enough healthy, mature sperm into the vagina; the egg must be fertilized within 24 hours after ovulation, and most sperm survive no more than one to two days. This merging of the egg and sperm is fertilization and mostly occurs in the upper portion of the fallopian tube. After fertilization, the cells begin to divide to form a zygote, then a morula, and finally a blastocyst. The blastocyst will gently travel down the fallopian tubes to the uterus and implant into the awaiting endometrium that has now gone through changes and become a decidua. The endometrial decidua will form into three layers: the decidua basalis (into which the chorionic villi grow), the decidua capsularis (which covers the blastocyst), and the decidua vera (which becomes the portion lining the uterus). Implantation occurs approximately 6–10 days after conception.

For the next steps involved in the process of conception, see the Fetal Development section

Fetal Development

"Fertilisation Phases of a Human Egg" by Campbell Biology, Pearson Education, used under CC BY SA 1.0/Cropped from original

"Three Cell Growth Types" by domdomegg , used under CC BY SA 4.0/Cropped from original

"Animated GIF of Cleavage, Morula and Gastrula Stages by Laird.sheldahl, used under CC BYSA 4.0/Cropped from original

"The First Few Weeks of Embryogenesis in Humans" by Jrockley, used under CC BY SA 2.0/Cropped from original

"ChorionicVillus" by BruceBlaus , used under CC BY SA 4.0/Cropped from original

"Early Formation of the Placenta by Tereza Cindrova-Davies and Amanda N. Sferruzi, used under CC BY SA 4.0/Cropped from original

Multiple Gestation

Multiple gestation can occur after fertilization of one egg that splits (identical multiples) or multiple eggs that were released and all were fertilized (fraternal multiples). They can share a placenta (monoamniotic) or have separate placentas (diamniotic). They can have one amniotic sac (monoamniotic) or separate amniotic sacs (diamniotic). When fetuses share a placenta, blood can be unequally transferred to one baby more than the other(s) leading to different growth. Known as a twin-to-twin transfusion, when this occurs, the smaller infant would be at risk of fetal demise and the obstetrician may choose to deliver the baby even if they are preterm. Fetuses that share an amniotic sac are at high-risk of tangling their umbilical cords leading to constriction of blood flow and fetal demise. Good instructions on monitoring fetal movement should be given to the mother and frequent antepartum testing should be done.