Pregnancy begins when a sperm fertilizes an egg. This usually occurs in the 2 weeks following the first day of the most recent menstrual period. At 3 weeks of pregnancy, a fertilized egg is too small to see.

In the first few weeks of pregnancy, a woman may have no symptoms. Some can sense that they are pregnant, but most do not suspect it until they miss the next period.

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

This feature is part of a series of articles on pregnancy. It provides a summary of each stage of pregnancy, 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.

For fertilization to occur, a sperm and egg must meetShare on Pinterest
For fertilization to occur, sperm must penetrate the egg.

Ovulation is the process of an ovary releasing an egg. Depending on the length of a person’s menstrual cycle, ovulation usually occurs around 2 weeks after the first day of the most recent period.

The medical community measures pregnancy from the first day of the last period, even if fertilization happens several days after the period has ended.

For example, if a woman’s most recent period began on June 1 and fertilization occurred on June 14, the next day the woman would be 2 weeks pregnant, even though fertilization just happened.

For fertilization to occur, sperm must enter the vagina. This usually, but not always, happens through sexual intercourse.

The sperm passes through the opening of the cervix and travels 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 either from a donor or a woman who wishes to become pregnant. In a laboratory, a specialist doctor works to fertilize the egg with donated sperm.

After a maximum of 6 days, the doctor transfers one or more fertilized eggs to the woman’s womb. The goal is for at least one fertilized egg to implant and develop.

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Chromosomes will determine the sex and other features of the unborn child.

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 make-up of the fetus, including its sex and inherited physical characteristics, such as:

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

They may also help shape factors such as personality, intelligence, and the risk of some health conditions.

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.

If the sperm contained an X chromosome, the fetus will be female (XX). If the sperm contained a Y chromosome, the fetus will be male (XY).

At 3 weeks pregnant, the fertilized egg, or zygote, measures 0.1 millimeters (mm), and it is too small to see.

The zygote 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.

At week 4, the blastocyst implants into the wall of the uterus. The blastocyst contains an inner and an outer part. The inner part will ultimately develop into the embryo. The outer part will become the placenta, an outer shell that will provide protection and nourishment to the growing embryo.

At this point, the baby is a cluster of cells measuring approximately 0.1–0.2 mm, or around the size of a poppy seed.

Around the beginning of week 5, doctors will likely start referring to the baby as an embryo, and it will be about the size of a sesame seed.

At 6 weeks, the baby is about the size of a lentil, or one-quarter of an inch long.

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A pregnancy test at week 3 is unlikely to be accurate. it is best to wait until one week after your next missed period.

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

  • eating a healthful 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, including recreational drugs, and certain foods and medications
  • getting enough 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 doctor can help modify or develop a healthful exercise plan.

During the first 6 weeks of pregnancy, there may be no indications of pregnancy.

However, soon after fertilization in some cases, and increasingly as time goes on, you may experience:

  • some spotting of blood
  • tiredness
  • breast tenderness
  • nausea and a sharper sense of smell
  • a need to pee more often

A pregnancy test is likely to produce an accurate result if a person takes it 1 week after the first day of the next expected period.