At the third week of pregnancy, the embryo is implanted into the uterus. At this stage, it is too small to see, and most people will experience no clear symptoms of pregnancy at this point.

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

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The medical community measures pregnancy from the first day of the last period. This means that, depending on the timing of fertilization, a person may not physically be pregnant during the first 2 or so weeks of defined pregnancy.

The implantation into the uterine wall signals the physical start of pregnancy. At this stage, doctors will refer to the fertilized egg as an embryo.

In this article, gain a detailed understanding of fertilization, and learn what happens in the first 5 weeks of pregnancy.

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Before fertilization, ovulation occurs. Ovulation is the process of an ovary releasing an egg. Depending on the length of a person’s menstrual cycle, ovulation usually occurs around 12–14 days after the first day of the most recent period.

For fertilization to then occur, sperm must enter the vagina, passing through the opening of the cervix and into the fallopian tubes.

There, the sperm will penetrate and fertilize an egg. Once fertilized, the egg travels down the fallopian tube toward the womb, or uterus, where it will implant in the uterine wall.

Doctors refer to the fertilized egg as an embryo after implantation. Beginning in the ninth week of pregnancy and until the pregnancy ends, doctors call the developing baby a fetus.

In-vitro fertilization

In-vitro fertilization occurs outside the body. The egg comes from a donor or a person who wishes to become pregnant. In a laboratory, a specialist doctor works to fertilize the egg with donated sperm.

The doctor transfers one or more fertilized eggs to the womb after either 3 or 5 days. The goal is for at least one fertilized egg to implant and develop.

This feature is part of a series of articles on pregnancy. It summarizes each pregnancy stage, what to expect, and insights into how your baby is developing.

Take a look at the other articles in the series:

First trimester: fertilization, implantation, week 5, week 6, week 7, week 8, week 9, week 10, week 11, week 12.

Second trimester: week 13, week 14, week 15, week 16, week 17, week 18, week 19, week 20, week 21, week 22, week 23, week 24, week 25, week 26.

Learn more in our dedicated pregnancay and parenthood hub.

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When the egg and sperm unite, they form a zygote. The zygote contains 46 chromosomes: 23 from the female and 23 from the male.

Chromosomes are tiny, threadlike structures. Each carries around 2,000 genes. The chromosomes determine the genetic makeup of the fetus, including its sex and inherited physical characteristics, such as:

  • hair color
  • eye color
  • blood type
  • height
  • build

A zygote inherits one sex chromosome, the X chromosome, from the egg. It also inherits a sex chromosome from the sperm. This may be either an X or Y chromosome.

Learn more about chromosomes and genes here.

At 3 weeks pregnant, the baby will measure 0.1 millimeters (mm), which is too small to see.

The embryo will spend several days making its way down the fallopian tube. During this time, it develops into a morula, a ball of 12–15 cells. As time goes on, it will become a bigger collection of cells called a blastocyst.

The following health strategies are essential for anyone planning or trying to become pregnant:

  • eating a healthy diet, and possibly taking supplements of folic acid and other vitamins
  • avoiding smoking and second-hand smoke
  • avoiding alcohol and other substances that can interfere with pregnancy
  • getting moderate exercise
  • notifying doctors about plans to become pregnant when discussing diagnoses or treatments

You may need to take a break from strenuous exercise during pregnancy, and a licensed professional can help modify or develop an exercise plan.

People may not experience clear pregnancy symptoms in the initial weeks after fertilization. However, some people may experience:

A pregnancy test is more likely to produce an accurate result if a person takes after the first day of the next expected period.

Learn more about how and when to take pregnancy tests here.

By the third week of pregnancy, a fertilized egg has typically traveled to the uterus and implanted in the uterine wall. At this stage, doctors will refer to the fertilized egg as an embryo.

Most people do not experience symptoms at this stage, but light spotting, nausea, and changes to urinary function may occur.