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AN78.1-5 | Second week of development — Gate Quiz
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Implantation of the blastocyst normally occurs on which post-fertilisation days?
Correct. Implantation begins on approximately day 6 (zona hatching and initial apposition) and is complete by day 10–12 post-fertilisation. In menstrual dating, this corresponds to days 20–24 of a 28-day cycle.
Blastocyst timeline: Fertilisation (day 0) → Morula (day 3–4) → Blastocyst (day 4–5) → Uterine cavity (day 5) → Zona hatching (day 5–6) → Implantation begins (day 6–7) → Implantation complete (day 10–12).
Days 1–5: blastocyst forming and travelling to uterus. Day 6: zona hatching. Days 6–10: implantation. Day 14: menstruation would have occurred if implantation failed.
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Which layer of the trophoblast is responsible for producing human chorionic gonadotrophin (hCG)?
Correct. hCG is produced by the SYNCYTIOTROPHOBLAST — the multinucleated outer layer formed by fusion of cytotrophoblast cells. hCG production begins from day 8–9 of development and can be detected in maternal serum/urine from day 10–12.
Syncytiotrophoblast functions: (1) invasion of endometrium; (2) hCG production (maintains corpus luteum → progesterone); (3) forms lacunae (maternal blood spaces for nutrient exchange); (4) produces progesterone (from week 8–10 onwards, taking over from corpus luteum).
Syncytiotrophoblast (outer, invasive layer) produces hCG. Cytotrophoblast (inner, mitotic layer) is the stem cell layer. The corpus luteum produces progesterone in response to hCG stimulation.
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What is the most common site of normal blastocyst implantation?
Correct. The most common site of normal implantation is the posterior wall of the body of the uterus, in the upper segment. The endometrium here is thick and well-vascularised during the secretory phase.
Normal implantation site: posterior > anterior wall of uterine body, upper segment. Abnormal sites (ectopic): tube (95%), ovary (3%), abdominal (1%), cervix (<1%). Site determines clinical risk — cervical and abdominal ectopics are the most dangerous.
Normal implantation: posterior wall of uterine body (upper segment). Anterior wall is less common but normal. Tube = ectopic. Cervix = cervical ectopic (rare, dangerous).
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A patient has a positive pregnancy test. Beta-hCG is 1,800 IU/L. Transvaginal ultrasound shows no intrauterine gestational sac. Repeat hCG 48 hours later: 2,200 IU/L. What is the MOST LIKELY diagnosis?
Correct. Normal IUP: hCG doubles every 48 hours (minimum 53% rise). 1,800 → 2,200 = 22% rise over 48 hours — well below normal. Combined with absent intrauterine sac, this strongly suggests ectopic pregnancy (or failing pregnancy). Immediate further management is required.
Clinical rule: beta-hCG should rise by at least 53% every 48 hours in a normal IUP. Below discriminatory zone (~1,500–2,000 IU/L) with no IU sac + subnormal hCG rise = ectopic until proven otherwise. Management: methotrexate (if criteria met) OR surgical (salpingectomy/salpingostomy).
hCG rise of 22% over 48 hours (expected minimum: 53%) + empty uterus on TVS = ectopic until proven otherwise. The discriminatory zone (~1,500–2,000 IU/L) is where a normal IUP should be visible on TVS.
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The hypoblast layer of the bilaminar embryonic disc gives rise to which structure?
Correct. The hypoblast (lower layer of bilaminar disc) gives rise to extraembryonic endoderm — the wall of the primary and secondary yolk sac. It does NOT contribute to the embryo proper. The epiblast gives rise to all three germ layers (via gastrulation).
Bilaminar disc key rule: EPIBLAST = source of embryo (all 3 germ layers via gastrulation) + amnion. HYPOBLAST = extraembryonic endoderm only (yolk sac wall) — contributes NOTHING to the embryo proper. This is a common exam trap.
Bilaminar disc: Epiblast (upper, columnar) → all germ layers + amnion. Hypoblast (lower, cuboidal) → extraembryonic endoderm (yolk sac) only — NOT to embryo proper.
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Why is a urine pregnancy test positive within days of implantation?
Correct. hCG is produced by the syncytiotrophoblast from day 8–9 of development (as it invades the endometrium and enters maternal blood vessels). It enters the maternal circulation → excreted in urine → detected by immunoassay pregnancy tests from approximately day 10–12.
Pregnancy test basis: hCG (a glycoprotein, LH-like structure). Produced by syncytiotrophoblast from day 8–9. Peaks at weeks 8–10 (60,000–100,000 IU/L), then falls. Maintained corpus luteum → progesterone → prevents menstruation. The beta-subunit of hCG is specific (alpha-subunit shared with LH, FSH, TSH).
hCG (from syncytiotrophoblast) enters maternal blood as invasion begins (day 8–9). Modern sensitive tests can detect hCG from day 10–12 post-fertilisation (day 24–26 of a 28-day cycle — 2 days before expected period).
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A 32-year-old woman with her third pregnancy (previous two were caesarean sections) presents at 30 weeks with painless bright red vaginal bleeding. TVS shows the placenta covering the internal os. Which implantation abnormality explains this?
Correct. Placenta praevia = low implantation of the blastocyst in the lower uterine segment, with the placenta partially or completely covering the internal os. Presenting symptom: painless antepartum haemorrhage (no uterine contractions — distinguishes from abruption).
Placenta praevia risk factors: prior CS (uterine scar), multiparity, multiple pregnancy, prior uterine surgery. Presentation: painless bright red APH (any trimester). Management: nil per vagina, hospitalisation, caesarean delivery. Rising CS rates in India make this increasingly common.
Placenta praevia: blastocyst implanted low → placenta covers cervical os → painless antepartum haemorrhage. Prior caesarean scars increase the risk of abnormal implantation in the lower segment.
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During the second week of development ('week of twos'), which pair of structures is formed from trophoblast differentiation?
Correct. The trophoblast differentiates into cytotrophoblast (inner, cellular, mitotic stem cell layer) and syncytiotrophoblast (outer, multinucleated, invasive, hCG-producing). This is one of the 'week of twos' pairs.
The 'week of twos' is a mnemonic for second-week development: 2 trophoblast layers (cyto + syncytio), 2 disc layers (epiblast + hypoblast), 2 cavities (amniotic + yolk sac), 2 extraembryonic mesoderm layers (somatic + splanchnic). All appearing simultaneously during days 7–14.
Week of twos pairs: trophoblast → cytotrophoblast + syncytiotrophoblast. Embryonic disc → epiblast + hypoblast. Fluid cavities → amniotic + yolk sac. Extraembryonic mesoderm → somatic + splanchnic.
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A woman with three prior caesarean sections has an anterior placenta praevia at 34 weeks. She is most at risk for which complication, directly related to abnormal trophoblast invasion?
Correct. Prior CS scars cause decidua deficiency at the scar site. When the placenta implants over the scar (as in anterior placenta praevia after CS), the trophoblast invades beyond the decidua into the myometrium (accreta) or beyond (increta/percreta). This is the accreta spectrum.
Placenta accreta spectrum risk with prior CS: 1 CS + praevia = 11–25%; 2 CS + praevia = 35–47%; 3 CS + praevia = 50–67%. India's rising CS rate (21.5% nationally, >50% private sector) is creating an epidemic of PAS. Antenatal diagnosis (MRI/USS) and planned delivery at a tertiary centre with blood bank and interventional radiology reduces mortality.
Placenta accreta spectrum (PAS): abnormal trophoblastic invasion beyond the decidua. Risk is exponentially increased with prior CS + placenta praevia. Major cause of peripartum hysterectomy and maternal mortality in India.
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During the second week of development, what is the significance of syncytiotrophoblast eroding maternal capillaries to form lacunae?
Correct. Syncytiotrophoblast erodes maternal capillaries → blood fills lacunae → trophoblast columns grow between lacunae → primary stem villi → primitive uteroplacental circulation. This is the beginning of maternal-foetal exchange (initially by diffusion across syncytiotrophoblast).
Lacunae (day 9–13): maternal blood fills spaces in syncytiotrophoblast → primary stem villi form between lacunae → primitive chorionic villi (week 3) → secondary villi (mesoderm core added) → tertiary villi (foetal vessels added, week 3). This is the embryological basis of the placenta.
Lacunae = spaces in syncytiotrophoblast filled with maternal blood. This is the first step in establishing the uteroplacental circulation — essential for early nutrient and gas exchange before the definitive placenta forms.
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