HANS-ULRICH OSTERWALDER / SCIENCE PHOTO LIBRARY HANS-ULRICH OSTERWALDER / SCIENCE PHOTO LIBRARY
Early stages of pregnancy. Diagram depicting early stages of pregnancy from conception to four weeks. A cross-section of the female reproductive system is seen in the foreground. The ovary releases the oocyte into the fallopian tube (ovulation). A sperm penetrates it and the parental chromosomes mix. The fertilised egg cell divides into a zygote and continues to increase in cell number. It enters the uterus and forms a hollow blastocyst and implants in the uterine lining. The placenta and umbilical cord are formed which nourish the foetus until birth.
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