Obstetric Topic 01. Conception, implantation and placentation

I. Conception

Definition

• Conception / fertilization: union of male and female pronuclear material → zygote.
• Usual site: ampulla of the fallopian tube.
• Timing: usually within 24 h after ovulation.

Ovulation and Fertilization

1. Ovulation → secondary oocyte leaves ovary and enters fallopian tube.
2. Oocyte: surrounded by zona pellucida and corona radiata; meiosis I completed + first polar body present.
3. Sperm capacitation + acrosome reaction → sperm penetrates corona radiata and zona pellucida.
4. Cortical reaction → blocks polyspermy.
5. Sperm entry → oocyte completes meiosis II → second polar body extruded.
6. Male + female pronuclei fuse → diploid zygote.

II. Preimplantation Development

Cleavage and Blastocyst

• Zygote undergoes cleavage while moving through the fallopian tube.
• Day 3: morula = compact mass of about 16 cells.
• Day 4-5: blastocyst enters uterine cavity and floats freely.
• Blastocyst parts:

• Embryoblast / inner cell mass → embryo.
• Trophoblast / outer cell mass → placenta and chorionic membranes.
• Blastocyst cavity → early extraembryonic space.

• Early preimplantation cells are totipotent → can form embryonic and extraembryonic tissues.

III. Implantation

Attachment and Invasion

• Begins around day 6-7 after fertilization/ovulation.
• Site: secretory endometrium, usually posterior-superior uterine wall.
• Blastocyst hatches from zona pellucida → apposition → adhesion → invasion.
• Trophoblast differentiates:

• Cytotrophoblast: inner cellular layer.
• Syncytiotrophoblast: invasive outer layer → erodes endometrium and maternal vessels.

• Syncytiotrophoblast produces hCG → maintains corpus luteum → progesterone support of endometrium.
• Week 2: lacunae form in syncytiotrophoblast and fill with maternal blood.

IV. Decidua and Placentation

Decidua

• Decidua: pregnancy-transformed endometrium.
• Decidua basalis: maternal part beneath embryo → basal plate of placenta.
• Decidua capsularis: covers the conceptus toward uterine cavity.
• Decidua parietalis/vera: remaining uterine lining.

Early Placenta

• Trophoblast + extraembryonic mesoderm → chorion.
• Chorionic villi form → fetal capillaries develop inside villi.
• Chorion frondosum: villi at decidua basalis → fetal part of placenta.
• Chorion laeve: smooth chorion where villi regress → chorionic membranes.
• Intervillous space contains maternal blood; fetal and maternal blood normally do not mix.
• Placental function: gas/nutrient exchange, waste removal, endocrine support, partial immune barrier.

Exam Sequence

• Ovulation → fertilization in ampulla → zygote → cleavage → morula → blastocyst → implantation → trophoblast invasion → chorionic villi → placenta.