Pregnancy

Overview – Pregnancy

Pregnancy is a dynamic physiological process initiated by fertilisation, followed by implantation, embryonic development, and placental formation. This article provides a comprehensive overview of early pregnancy — from clinical signs and hormonal markers to blastocyst implantation and placentation. Understanding this timeline is crucial for interpreting antenatal tests, managing early pregnancy issues, and preparing for OSCE stations.

Aetiology

• Unprotected sexual intercourse
• In vitro fertilisation (IVF)
• Artificial insemination (AI)

Clinical Features

Symptoms

• Amenorrhoea
• Nausea and vomiting
• Heartburn or reflux
• Breast tenderness
• Urinary frequency
• Constipation
• Fatigue

Signs

• Softening of the cervix (by 4–6 weeks)
• Chadwick’s sign: Bluish vaginal mucosa due to vascular engorgement
• Uterine enlargement
• Breast enlargement and areolar darkening

Diagnosis of Pregnancy

β-hCG Testing

• Serum: Positive ~9 days post-conception
• Urine: Positive ~28 days post-conception

Ultrasound Findings

• Transvaginal:
  • 5 weeks: Gestational sac visible
  • 6 weeks: Foetal pole visible
  • 7–8 weeks: Foetal heartbeat detectable
• Transabdominal:
  • 6 weeks: Detectable intrauterine pregnancy

Physiological Amenorrhoea in Pregnancy

• Trophoblasts secrete β-hCG, maintaining the corpus luteum → continuous oestrogen and progesterone secretion → prevents menstruation.
• Trophoblasts → Chorion → continues β-hCG production.
• Placenta takes over hormone production around the end of the first trimester.

Low β-hCG for gestational age may indicate ectopic pregnancy, miscarriage, or incorrect dating.

Fertilisation & Implantation

Fertilisation Window

• Oocyte: Viable up to 24 hrs post-ovulation.
• Sperm: Viable up to 48 hrs in female tract.
• Fertilisation typically occurs in the ampulla of the fallopian tube.

Sperm Capacitation & Penetration

1. Sperm are capacitated by female tract secretions.
2. Corona radiata penetrated by hyaluronidase.
3. Zona pellucida digested via acrosomal enzymes (requires hundreds of sperm).
4. One sperm fuses with oocyte membrane → triggers cortical reaction → prevents polyspermy.
5. Secondary oocyte completes meiosis II → forms ovum + second polar body.
6. Sperm loses tail and swells → male pronucleus.
7. Male and female pronuclei fuse → zygote forms.
8. Mitosis begins (cleavage).

From Zygote to Blastocyst Implantation

Cleavage & Blastocyst Formation

• Zygote undergoes rapid mitosis → morula.
• Blastocyst forms as fluid accumulates inside morula.
• Zona pellucida degrades → blastocyst hatches.

Implantation (6–12 Days Post-Ovulation)

• Requires receptive endometrium (secretory phase).
• Trophoblasts bind to endometrial matrix, may “float” and reattach if initial contact fails.
• Two trophoblast layers form:
  • Cytotrophoblast (inner)
  • Syncytiotrophoblast (outer, invasive)
• Syncytiotrophoblast digests uterine tissue → embryo gains access to maternal nutrients.
• Implantation completes by ~Day 12 → trophoblasts secrete β-hCG to maintain corpus luteum.

Placentation

Chorion Formation

• Extraembryonic mesoderm + cytotrophoblast → chorion.
• Chorionic villi develop and invade maternal blood pools (lacunae).

Decidualisation of Endometrium

• Decidua basalis: Site of placental formation (maternal side).
• Decidua capsularis: Surrounds embryo; eventually degenerates as uterus expands.

Placental Functions

• Nutritional: Glucose, amino acids, fatty acids
• Respiratory: Oxygen and carbon dioxide exchange
• Excretory: Waste removal
• Endocrine: Secretes hCG, progesterone, oestrogen

Embryonic Development Overview

Germ Layer Formation

• Ectoderm: Skin, nervous system, amnion
• Mesoderm: Heart, vessels, muscle, bone, connective tissue
• Endoderm: GI, respiratory, urinary epithelium, yolk sac

By 8 weeks, all organ systems have begun to form.

Summary – Pregnancy

Pregnancy begins with fertilisation and proceeds through cleavage, implantation, placentation, and embryonic development. β-hCG plays a critical role in maintaining early gestation and signals pregnancy on clinical testing. Understanding these milestones is vital for managing antenatal care. For a broader context, see our Obstetrics Overview page.