4 Weeks Pregnant: Baby Size, Symptoms and What to Do

Your baby this week

Your baby is about the size of a poppy seed and has just settled into the lining of your uterus. A fluid-filled sac surrounds the embryo, and a tiny yolk sac supplies nourishment for now. The outer layer of cells will go on to form the placenta.

Pregnancy weeks are counted from the first day of your last period, so at 4 weeks conception happened only about two weeks ago. After implantation, the early placenta starts making hCG, the hormone a home test picks up. hCG rises quickly in early pregnancy, so even a faint line usually counts as positive if it shows up around the time your period is due. If you're unsure, test again in two or three days with first-morning urine. An ultrasound would show at most a tiny gestational sac now, which is why the first prenatal visit is usually a few weeks away. Folic acid matters most at this stage because the brain and spinal cord are forming: 400 micrograms a day is the standard advice. If you've had an ectopic pregnancy before, or you have sharp pain on one side, don't wait for a routine visit.

By the numbers

Week 4 covers days 28 to 34 counted from the first day of your last period, which is 10% of a 40-week pregnancy. About 252 days remain until a 40-week due date. By date of conception, your baby is about 2 weeks old.

Pregnancy is dated from the first day of your last period, so conception happens around week 2. Your due date is 280 days (40 weeks) from that day, and an early ultrasound may adjust it.

Your body

This is usually when your period is late and a home pregnancy test can turn positive. Mild cramping or light spotting can happen. Call a doctor if you have heavy bleeding or strong pain.

Tests and to-dos this week

Start a daily prenatal vitamin with folic acid if you haven't already. Call an OB-GYN or midwife to set up your first prenatal visit. Skip alcohol and smoking from now on, and check with a provider before taking any medication.

No routine test falls in this exact week; your provider schedules your regular checkups.

Coming up: First prenatal visit (from week 6). Full prenatal visit schedule

Tips for the first trimester

• Take 400 micrograms of folic acid a day; your provider may advise a higher dose if neural tube defects run in your family.
• Avoid alcohol, smoking, raw or undercooked meat, fish and eggs, and unpasteurized dairy.
• For nausea, try small, frequent meals and sipping fluids; call your provider if you can't keep anything down.
• Check every medicine and supplement you take with your provider or pharmacist before continuing.

Last week and next week

Next week (5): Still about the size of a sesame seed, your embryo is busy.

Frequently asked questions