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Fetal Development Index

Conception occurs when a woman’s egg is fertilized by a man’s sperm. The union of egg and sperm usually occurs in a woman’s fallopian tube. The cell, which now contains a complete set of genetic material, begins to divide into more cells while traveling down the fallopian tube to the uterus. This journey takes about a week.

By the eighth day after fertilization, the ball of cells has begun to attach itself to the wall of the uterus to get nutrients to continue growth. The growing body of cells is called an embryo from the time of attachment in the uterus until the ninth week of growth.

After the ninth week, the embryo becomes a fetus.   Fetal age is described in terms of the number of weeks from conception and the number of weeks from the last menstrual period. 

Health care providers use date from the last menstrual period and an ultrasound examination to estimate a due date. Fetal size is measure by weight and by length. Length is measured from the top of the head (crown) to the butt (rump), called crown-rump length (CRL). Note that fetuses may develop at different rates.

The descriptions of fetal development are not intended to be complete. For more information on fetal development, please see the reference list.

Week 4
Week 6
Week 8
Week 10
Week 12
Week 14
Week 16
Week 18
Week 20
Week 22
Week 24
Week 26
Week 28