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Implantation & Fetus

  • Changes within the endometrium mark the so-called window of implantation seen on days 20-24 of menstrual cycle. Close examination of the surface epithelial cells during this time has shown an increase in microvilli and cilia on cell surface into the lumen. These protrusions, termed pinopodes, are an important event in preparation for blastocyst implantation.
  • The morula after spending about 3 days in the tube enters the uterine cavity via the narrow ostium (1 mm) on the fourth day in the 12-16 cell stage.
  • Implantation occurs in the endometrium on the anterior or posterior wall of the body near the fundus on the sixth day following fertilization (corresponding to the 20th day of the menstrual cycle).
  • The deeper penetration of the human blastocyst is called interstitial implantation, which happens by approximately the 13th day after fertilization.

Important Events

Following Fertilization

'0' hour


4th day

16 cell stage

Morula enters uterine cavity

5th day


7th day

Interstitial implantation occurs

21st - 22nd day

Placenta fully established / Fetal circulation established and heart formed

8 weeks

Internal gonads formed

10-12 weeks

Swallowing starts

12 weeks

External genitalia formed

11 weeks

Fetal breathing movements

12 weeks

Urine formation occurs


Growth period

Seen Form

Important Points

Ovulation Period

Fertilisation-2 weeks


Embryonic Period

3 weeks-8 weeks

Most teratogenic Period

Fetal Period

>8 weeks and till delivery



Important Points:

The fetal circulation is established by 21 st days after fertilization.

  • By the end of the 12th week of pregnancy, when the uterus usually is just palpable above the symphysis pubis, the crown-rump length of the fetus is 6-7 cm. The fetus begins to make spontaneous movements.
  • By the end of the 16th week, the crown-rump length of the fetus is 12 cm, and the weight is 110 g. Gender can be correctly determined by inspection of external genitalia by 14 weeks.
  • By the end of the 24th week, the fetus weights about 630 g.
  • By the end of the 28th week a crown-rump length of about 25 cm is attained and the fetus weights about 1100 g.
  • At the end of 32 gestational weeks, the fetus has attained a crown-rump length of about 28 cm and a weight of about 1800 g.

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