Week 4 of Pregnancy

Baby Size Comparison in Week 4

In week 4 of pregnancy your baby measures about 2 mm long and weighs about Under 1 gram: roughly the size of Poppy seed. Remember these are approximate averages from standard growth charts; every baby grows at their own pace, and your doctor follows your baby's personal growth curve across visits, not a single number. If an ultrasound measures a little bigger or smaller than average, that is usually normal as long as growth continues at a steady rate.

Baby Development in Week 4

The blastocyst has reached the uterus and begun implanting into its blood-rich lining in a delicate process that takes days; it first attaches, then digs itself a place inside the lining until it is completely buried in it. From this point, the pregnancy hormone (hCG) starts rising in your blood; it is the hormone that pregnancy tests detect.

The cells begin differentiating into two layers: an inner layer that will become the embryo, and an outer layer that will become the placenta, the umbilical cord, and the amniotic sac. The primitive neural tube also begins forming, the foundation from which the brain and spinal cord will later grow. By the end of this week, your period is late, the first clear sign prompting you to take a test.

After implantation, the pregnancy hormone in your blood roughly doubles every 48 to 72 hours in a healthy pregnancy, and this steady doubling is what reassures doctors in early pregnancy. The yolk sac forms and nourishes the embryo temporarily until the placenta is complete, and the first umbilical cord begins to take shape. The quantitative blood test for the hormone (beta-hCG) is more accurate than a home test and detects pregnancy days before the missed period. Any heavy bleeding or sharp pain on one side with dizziness calls for an urgent visit to rule out an ectopic pregnancy.

Your Body Changes in Week 4

The missed period is the most prominent symptom this week. Before it, you may notice very light implantation bleeding lasting a day or two, pink or brown in color; it is normal and unlike period bleeding. Breast tenderness and a feeling of fullness begin, along with unusual fatigue and a frequent urge to urinate from increased blood flow to the pelvis.

Some women feel mild cramps resembling period cramps from implantation and uterine stretching. Emotionally, joy mixes with anxiety at seeing the two lines on the test; completely normal feelings at the start of this journey.

The quantitative blood test confirms pregnancy and roughly estimates its age from the hormone level. Tell your doctor about any medications you take regularly so their safety can be assessed. Start organizing your sleep; good sleep supports immunity and hormones. And avoid lifting heavy things and standing for long without need during this sensitive stage.

Practical Tips for Week 4

• Take a home pregnancy test in the morning one day after your missed period; morning urine is the most concentrated with the hormone.
• Once the result is positive, book an appointment with your doctor to confirm the pregnancy with ultrasound and lab tests.
• Keep taking folic acid and avoid raw foods such as sushi and undercooked meats.
• Do not worry about light implantation bleeding, but tell your doctor about any heavy bleeding or severe pain.

Frequently Asked Questions About Week 4