GUIDE
Your baby is the size of a poppy seed — and a pregnancy test can now detect the pregnancy.
This is the week most people discover they are pregnant. Your period is late, hCG levels are rising rapidly, and a home pregnancy test should be able to detect the hormone in your urine. Meanwhile, the tiny embryo has fully implanted in your uterine wall and is beginning to form the structures that will become the placenta, amniotic sac, and yolk sac.
Be prepared from day one
Your baby is about the size of a poppy seed — roughly 1 millimeter long. At this stage, the embryo is a two-layered disc of cells called the bilaminar disc, which will eventually give rise to all of the body's organs and tissues.
Implantation is now complete. The trophoblast cells (the outer layer of the blastocyst) have burrowed deep into the uterine lining and are forming the early placenta — the chorionic villi that will eventually handle the exchange of nutrients and waste between you and your baby. The amniotic sac is beginning to form around the embryo, and the yolk sac is providing nourishment until the placenta takes over.
The embryo's cells are beginning to differentiate into three primary layers: the ectoderm (which will become the skin, nervous system, and brain), the mesoderm (which will become the heart, muscles, bones, and circulatory system), and the endoderm (which will become the lungs, liver, and digestive system). This process — called gastrulation — is one of the most critical events in embryonic development.
For a look at what happened last week, see our week 3 guide.
The most noticeable change this week is your missed period. For many people, this is the first concrete sign that something is different. Behind the scenes, hCG levels are doubling roughly every 48-72 hours, which is what makes the pregnancy test line appear.
Progesterone levels are climbing, and you may feel its effects: fatigue, bloating, breast tenderness, and mood changes. Progesterone relaxes smooth muscle throughout your body, which can slow digestion and cause that heavy, bloated feeling. It also has a mild sedative effect, which is why many newly pregnant people feel exhausted even though they are getting enough sleep.
Your blood volume is already beginning to increase — a process that will continue throughout pregnancy, eventually reaching about 50% more blood than pre-pregnancy levels. Your kidneys are working harder, filtering more blood, and you may notice you need to urinate more frequently. These changes are subtle at week 4, but they are underway.
Every pregnancy is different. You may experience all, some, or none of these symptoms.
This is the week of confirmation and first steps. Once you see that positive test, there are a few practical things to set in motion.
If your period is late, a home pregnancy test is the logical first step. Test with your first morning urine for the highest concentration of hCG. Most modern tests are very accurate by the day of your expected period. Two lines, a plus sign, or the word 'pregnant' — however your test displays it — means hCG has been detected.
Schedule your first prenatal appointment. Most practices will see you between weeks 8 and 10 for the initial visit, which typically includes an ultrasound, blood work, and a thorough health history. Some providers may want to see you sooner if you have a history of complications.
Now that you are pregnant, certain foods pose risks. Avoid raw or undercooked meat, fish, and eggs; unpasteurized dairy and juices; high-mercury fish (swordfish, king mackerel, tilefish, shark); deli meats unless heated to steaming; and soft cheeses made with unpasteurized milk.
A prenatal vitamin with at least 400-800 mcg of folic acid, plus iron, calcium, and DHA, supports your baby's development through the critical early weeks. If prenatal vitamins cause nausea, try taking them at bedtime with a small snack.
Tracking symptoms, questions for your provider, and milestones from the beginning gives you a valuable record. Many parents find it helpful to look back on these notes later.
When in doubt, call your provider. No question is too small during pregnancy.
Next week, the embryo's heart will begin to form and the neural tube will start developing. You may also begin to experience the earliest hints of morning sickness as hCG levels continue their rapid climb.
Next up: Week 5. Review our foods to avoid during pregnancy guide and make sure your prenatal vitamin is dialed in. Just got a positive result? Our positive pregnancy test — now what guide covers every first step. And if the line was faint, read about what a faint line on a pregnancy test really means.
This guide is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider with any questions about your pregnancy.
