Embryo vs. Fetus: Differences between Stages Week by Week

• The terms embryo and fetus both refer to the developing baby inside the mother’s womb (uterus).
• The distinction between embryo and fetus is made based on gestational age.
• An embryo is the early stage of human development in which organs are critical body structures are formed.
• An embryo is termed a fetus beginning in the 11th week of pregnancy, which is the 9th week of development after fertilization of the egg.
• A zygote is a single-celled organism resulting from a fertilized egg. The zygote divides to become a ball of cells that eventually implants in the wall of the uterus.
• This ball of cells, known as a blastocyst, develops into the embryo and placenta.
• Doctors date pregnancy from the first day of the last menstrual period.

Doctors define pregnancy according to the date of a woman’s last menstrual period.

• The first two weeks of pregnancy are counted as the time prior to ovulation, in which the body is preparing to release an egg.
• Week #3 begins with release of an egg, or ovulation. If the egg is fertilized by a sperm cell, it is known as a zygote. The zygote divides and becomes a collection of cells known as a blastocyst.
• In week #4 of pregnancy, the blastocyst implants in the wall of the uterus and develops into the placenta and embryo. The blastocyst is considered an embryo at the point when the amniotic sac develops (by about day 10 to 12 after fertilization, or at the start of week #5 of pregnancy).
• An embryo represents the early stage of human development, roughly corresponding to the 5th-10th weeks of pregnancy.

After the embryonic period has ended at the end of the 10th week of pregnancy, the embryo is now considered a fetus. A fetus is a developing baby beginning in the 11th week of pregnancy.

The embryonic period of development is the stage in which most organs are formed. Organs begin to form, and the embryo elongates and starts to assume a human-like shape. The heart and main blood vessels are formed, and the heart starts beating by the 6th week. The primitive brain and spinal cord (neural tube) also begin to form.

By 12 weeks of pregnancy almost all of the baby’s organs are formed, including the arms and legs. The brain and spinal cord, however, continue to develop throughout pregnancy. The baby is about 2 inches long at the end of the embryonic period.

Most birth defects occur during the embryonic period, the critical time when organs are forming. The embryonic period is the time of greatest susceptibility to the negative effects of drugs, radiation, and viral infections.

The fetal period is a time of growth of the developing baby. The organs and structures formed in the embryonic period continue to grow and develop.

The second trimester begins in the 13th week. By about 14 weeks of pregnancy, the gender of the fetus can be identified. Women can begin to feel movement of the fetus somewhere between the 16th and 20th weeks of pregnancy. The baby grows to about 2 pounds during the second trimester.

You should set up an appointment with your doctor or midwife as soon as you have a positive pregnancy test result. At the initial visit, your doctor or midwife will

• Discuss your medical history and any effect this may have on your pregnancy.
• Estimate your due date.
• Perform a physical examination.
• Order lab work to look for any infections and to determine your blood type, immunity against certain infections, and your red blood cell count
• Discuss any medications you are taking, and whether you will need to alter these during pregnancy and discuss the need for vitamins or supplements during pregnancy
• Discuss expected weight gain, activity level, and nutrition for a healthy pregnancy
• Provide contact information to call in case you have any questions, or are worried that something is wrong
• Answer any questions you may have
• Set up your next appointment and discuss how often you should visit your doctor or midwife.

The greatest risks of miscarriage are in the very early stages of pregnancy. An estimated 25% or more of pregnancies end in the very early stages, many before a woman even knows that she is pregnant or has missed a menstrual period. Most other miscarriages happen in the first 12 weeks of pregnancy when the embryo is developing.