Four Weeks Pregnant: What to Expect
The fourth week of pregnancy is when you will see positive results on a pregnancy test.
Credit: Hadi Djunaedi | Shutterstock

This is part of a Live Science series of articles on the weekly changes that a pregnant woman's body goes through as it adapts to the growing needs of the fetus inside her.

In the fourth week of your pregnancy (measured from the first day of your last period), you will begin to register positive results on pregnancy tests and be able to confirm that you are pregnant. For the sake of accuracy, wait until the end of the week to take any home pregnancy tests. If the test comes back positive, congratulations! You should make an appointment with your health care provider for a prenatal checkup. If the test is still negative, take another test at five weeks. 

Most practitioners don't see patients until they are eight weeks along, so you may need to wait a few weeks before actually seeing a specialist. However, if you have had a high-risk pregnancy or a history of problems in giving birth, you should see the health care provider sooner than that. 

This early in the pregnancy, there won't be any major outward changes in your body, though the basal body temperature — your body temperature when you are completely at rest — will be high. You may notice some mild cramping. Some women will experience an increase in vaginal discharge or spotting, caused by the egg burrowing into the uterine lining. An absence of spotting is not a cause for concern, as most women do not spot during the first few weeks of pregnancy. 

Some pregnancy symptoms can start at this point, including fatigue and exhaustion. Higher hormone levels direct more blood flow to your breasts, causing them to be tender and sore. The rapidly rising levels of estrogen could even cause a heightened sense of smell. Though morning sickness doesn't usually begin for a few weeks, some women may experience nausea or vomiting at this stage. You may start craving certain foods, and foods that you previously enjoyed will start to taste different.

At four weeks, the blastocyst — a tiny group of embryonic cells — would have already made the journey from your fallopian tube into your uterus and implanted along the lining. After it implants, it will have developed into an embryo and a placenta, which will produce hormones like human chorionic gonadotropin (hCG). The hCG alerts your ovaries to stop releasing eggs and increases the production of estrogen and progesterone. 

The placenta, made up of two layers, provides nutrients and oxygen to the embryo that will begin functioning at the end of the week. The amniotic sac and yolk sac will form, which produce the red blood cells and deliver nutrients until the placenta has fully developed.

The embryo, now about 2 millimeters (0.079 inches) long, according to the American Pregnancy Association, has three layers:

  • The ectoderm (outer layer) will become the nervous system, including the brain, as well as skin, hair, nails, eyes and teeth.
  • The mesoderm (middle layer) will develop into the circulatory system, heart, sex organs, skeleton, connective tissues and muscles.
  • The endoderm (inner layer) will become the lungs, as well as the lining of the gastrointestinal tract, the liver, pancreas and thyroid gland.

Rudimentary blood begins to move through the main vessels. Arm and leg buds may begin developing at this point, but at that small size, they are not yet distinguishable. 

At this point and throughout the pregnancy, you should avoid alcohol, caffeine, recreational drugs and smoking. The fetus is taking in everything that you do, and you don't want to inhibit any fetal development. Certain medications and foods should also be avoided — consult with your health care provider to make sure none of your medications are harmful to the fetus.

This is when you should start developing good eating habits. To help the fetus grow, What to Expect, a pregnancy advice website, suggests eating three servings of lean protein on a daily basis. This will help tissue develop. One serving is roughly 3 ounces (85 grams), or the size of a deck of playing cards. This could be lean beef, chicken, legumes or tofu. Lean red meat will also help with your iron intake, which you need to support the increased blood volume. Add some foods rich in vitamin C, like oranges and berries, to help with iron absorption. 

Folic acid is very important at this stage, as is calcium. recommends getting 600 micrograms of folic acid a day. Prenatal vitamins can provide that much; talk to your health care provider about adding them to your diet. Leafy greens like spinach deliver both of these nutrients, which help to promote bone development and avoid birth defects. Citrus foods are also naturally high in folate, so a calcium-enriched orange juice is a great addition to any breakfast. 

The American Pregnancy Association recommends talking to your health care provider before beginning or continuing an exercise routine, but exercise is very important at this stage. Focus on cardiovascular health to keep your heart healthy and strength training to help lessen the potential for lower-back pain throughout the pregnancy.