February 19, 2025
Fetal development – UF Health

Definition

Learn how your baby is conceived and how your baby develops inside the mother’s womb.

Alternative Names

Zygote; Blastocyst; Embryo; Fetus

Information

WEEK BY WEEK CHANGES

Gestation is the period of time between conception and birth when a baby grows and develops inside the mother’s womb. Because it’s impossible to know exactly when conception occurs, gestational age is measured from the first day of the mother’s last menstrual cycle to the current date. It is measured in weeks.

This means that during weeks 1 and 2 of pregnancy, a woman is not yet pregnant. This is when your body is preparing for a baby. A normal gestation lasts anywhere from 37 to 42 weeks.

Week 1 to 2

  • The first week of pregnancy starts with the first day of a woman’s menstrual period. She is not yet pregnant.
  • During the end of the second week, an egg is released from an ovary. This is when you are most likely to conceive if you have unprotected intercourse.

Week 3

  • During intercourse, sperm enters the vagina after the man ejaculates. The strongest sperm will travel through the cervix (the opening of the womb, or uterus), and into the fallopian tubes.
  • A single sperm and the mother’s egg cell meet in the fallopian tube. When the single sperm enters the egg, conception occurs. The combined sperm and egg is called a zygote.
  • The zygote contains all of the genetic information (DNA) needed to become a baby. Half the DNA comes from the mother’s egg and half from the father’s sperm.
  • The zygote spends the next few days traveling down the fallopian tube. During this time, it divides to form a ball of cells called a blastocyst.
  • A blastocyst is made up of an inner group of cells with an outer shell.
  • The inner group of cells will become the embryo. The embryo is what will develop into your baby.
  • The outer group of cells will become structures, called membranes, which nourish and protect the embryo.

Week 4

  • Once the blastocyst reaches the uterus, it buries itself in the uterine wall.
  • At this point in the mother’s menstrual cycle, the lining of the uterus is thick with blood and ready to support a baby.
  • The blastocyst sticks tightly to the wall of the uterus and receives nourishment from the mother’s blood.

References

Feigelman S, Finkelstein LH. Assessment of fetal growth and development. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 20.

Ross MG, Desai M, Ervin MG. Fetal development, physiology, and effects on long-term health. In: Landon MB, Galan HL, Jauniaux ERM, et al, eds. Gabbe’s Obstetrics: Normal and Problem Pregnancies. 8th ed. Philadelphia, PA: Elsevier; 2021:chap 2.

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