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HomeHealth articlespregnancyWhat Happens at Four Weeks of Pregnancy?

Week Four of Pregnancy: A Comprehensive Guide

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Week four marks the first few weeks of the first trimester of pregnancy. Read the article to know more regarding week four of the gestation period.

Medically reviewed by

Dr. Daswani Deepti Puranlal

Published At June 16, 2023
Reviewed AtJune 16, 2023


Week four of the pregnancy marks the completion of the first month of pregnancy, even though the women may or may not know about it. In the case of the regular cycle consisting of around 28 days, the pregnant women may have missed their period at week four of the pregnancy. Week four of pregnancy is the embryonic period which starts after the implantation of the embryo and lasts up to week ten. At this stage, the future baby is called an embryo.

Embryogenesis refers to the processes of divisions and differentiation of the cells. At week four, during the embryonic period, the organs of the fetus grow. Most vital organs of the developing fetus, including the kidneys, brain, liver, and lungs, develop at this stage. These organs later will continue to mature during the whole gestation period.

What Is the Growth of an Embryo at Week Four?

Week four is also called an organogenic period. This is a crucial period of development for the organs of the embryo because, during the first three weeks of the pregnancy, teratogens (any agent that can cause or increase the risk of a birth defect in the baby) do not affect the embryo. During the four weeks of pregnancy, the use of teratogens can affect the growth and functions of the developing baby.

During this important embryonic phase of the pregnancy, the growth and development of the embryo occur within the lining of the uterus. There are two layers that are functional at this stage: the outer layer cells, which form links with maternal blood supply, and the inner cells, which form three separate layers of the embryo as follows:

  • The inner layer of the embryo forms the lungs, gastrointestinal system, stomach, and bladder.

  • The middle layer of the embryo forms the heart, blood vessels, bones, and muscles.

  • The outer layer of the embryo forms skin, nails, the brain, and the nervous system.

At four weeks of pregnancy, the embryo consists of two cellular layers, the epiblast and the hypoblast, which eventually develop into organs and other body parts of the baby. Two other structures which develop at week four are the yolk sac and the amnion. The amnion is a cavity filled with amniotic fluid. Amniotic fluid surrounds and protects the growing embryo. The yolk sac will form a vascularization and provide nourishment to the embryo until the formation and function of the placenta starts, which will develop from the outer layer of the amnion that surrounds the embryo.

At week four of the pregnancy, the size of the embryo is that of a poppy seed. At the end of week four, the embryo is about 0.08 inch (2 millimeter) long.

What Are the Changes Occurring at Week Four in the Body of a Pregnant Woman?

Pregnant women may not notice any differences in their bodies and probably will feel no symptoms at week four of the pregnancy. The woman will not see any physical changes on their belly. However, there are many hormonal changes that occur at week four of the pregnancy. Hormones are essential for preparing the female body for the implantation of a fertilized egg and its growth and nourishment. The corpus luteum of the uterus lining and, later in the pregnancy, the placenta releases progesterone hormone, which creates and maintains a thick layer of endometrium inside the uterus to provide a nourishing environment for the implantation and growth of the embryo.

Some women may experience very slight bleeding or spotting at week four of the pregnancy, and at the start of week four of the pregnancy, implantation may occur. After the implantation process is complete, the growing placenta will release beta-human chorionic gonadotropin hormone. The women may take a home pregnancy test at week four which may come positive if some level of beta human chorionic gonadotropin hormone gets detected in the test. While in some women, week four of the pregnancy may still be early to have a positive home pregnancy test.

What Are the Pregnancy Symptoms at Week Four of the Pregnancy?

If the menstrual cycle of the pregnant woman is regular, then the most reliable and first symptom of the pregnancy is a missing period after having sexual intercourse without using any form of protection or in case of failed contraception. However, there are some other causes also for a delayed or missed period. The only way to confirm the pregnancy at week four is to take a pregnancy test as they are cost-effective and easily accessible. However, the women with missed or delayed periods at week four may also opt for gynecological consultation and check-up. If the woman is pregnant, then at week four of the pregnancy, they may notice the following symptoms of early pregnancy;

  • Nausea.

  • Vomiting.

  • Aversion to food.

  • Fatigue or tiredness.

  • Sore or tender breasts.

  • Frequent urination.

What Are the Ultrasound Changes at Week Four of Pregnancy?

At week four of the pregnancy, the blastocyst that is formed by fertilization of the female ovum by sperm splits into the placenta and embryo. Hence, an ultrasound of the uterus at this stage will not show an embryo. Even the most experienced sonographer operating the most advanced transvaginal ultrasound equipment may not be able to detect live pregnancy at week four. Sometimes, the doctor can get hints of pregnancy at week four on ultrasound as they can see ovulation has occurred by looking for the corpus luteum. The doctor may also detect the thick and bright lining of the uterus, which reflects the changes due to the pregnancy hormones. However, the pregnancy should be at least five to six weeks along to confirm it on the ultrasonography. Hence, in most countries, the first ultrasonography scan of a pregnant woman is recommended between weeks eight and fourteen.

What Is the Clinical Significance at Week Four of the Pregnancy?

An important clinical significance at week four of pregnancy is initiating folic acid supplements, if not already given, as pre-pregnancy planning to avoid neural tube defects. Neural tube defects are birth defects of the nervous system that happen as a result of the failure of the neural tube to close completely.

Another important clinical significance is a group of genes called Hox genes. Hox genes play an important role in the development of the body plan. In case of mutations in these genes, the body parts may develop at an incorrect location.


At week four of the pregnancy, the woman may have the first sign of pregnancy, which is a missed period. A home pregnancy test at this time may come positive in case of pregnancy. The women might not experience the symptoms of early pregnancy yet, but there are many changes occurring inside their bodies, including hormonal changes and embryonic development. The end of the four weeks refers to the completion of one month of pregnancy. The doctors recommend pregnant women at this stage take prenatal supplements, including folic acid, adopt a healthy diet and lifestyle, stop smoking, and drink alcohol.

Dr. Daswani Deepti Puranlal
Dr. Daswani Deepti Puranlal

Obstetrics and Gynecology


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