Week 4 of Pregnancy: Your Baby Is the Size of a Poppy Seed

TL;DR

• Your baby is the size of a poppy seed, about 2mm
• The neural tube — which becomes the brain and spinal cord — is forming
• You might notice a missed period, fatigue, and breast tenderness
• Schedule your first prenatal appointment

Your Baby This Week

At four weeks, your baby is about the size of a poppy seed — roughly 2mm — and is officially called an embryo. It might be hard to imagine that something so incredibly tiny is already developing structures that will become a brain, a heart, and a spinal cord, but that's exactly what's happening.

The most significant development this week is the formation of the neural tube. This structure will eventually become your baby's brain and spinal cord, making it one of the first and most critical things to develop. This is why folic acid is so important in early pregnancy — it directly supports neural tube formation and helps prevent defects like spina bifida.

Your baby is organized into three distinct layers of cells, each with a specific role. The ectoderm (outer layer) will form the nervous system, skin, hair, and eyes. The mesoderm (middle layer) will become the heart, circulatory system, bones, and muscles. The endoderm (inner layer) will develop into the lungs, liver, and digestive system.

The placenta is also taking shape this week, burrowing deeper into the uterine wall and building a network of blood vessels that will connect your blood supply to your baby's. This incredible organ will be your baby's lifeline for the next eight months, providing oxygen and nutrients while removing waste.

Your Body This Week

This is the week when many people realize they might be pregnant. Your period is late — or missing entirely — and that's often the first unmistakable clue. A home pregnancy test taken now should give you a reliable result, since hCG levels are usually high enough to detect.

If you're newly seeing those two lines (or that "Pregnant" result on a digital test), take a moment. Whether this pregnancy was carefully planned or a complete surprise, it's okay to feel whatever you're feeling — excitement, shock, joy, terror, or all of those at once.

Physically, you might notice that the symptoms from last week are intensifying. Fatigue can feel overwhelming. Breast tenderness may increase. Some people start experiencing their first waves of nausea, though full-blown morning sickness usually doesn't kick in until weeks 6-8.

You might also notice that you need to urinate more frequently. This is caused by increased blood flow to your kidneys and rising hCG levels. It's one of those early pregnancy symptoms that starts now and, honestly, doesn't really let up until after delivery.

Common Questions at Week 4

I just got a positive pregnancy test. What do I do now?

First, congratulations. Take a breath. When you're ready, call your healthcare provider to schedule your first prenatal appointment. Most providers will see you around weeks 8-10, but calling now lets you get on the calendar and ask any pressing questions. In the meantime, start taking a prenatal vitamin if you aren't already, and review any medications you're currently taking with your provider.

Is it normal for a pregnancy test line to be faint?

Yes. A faint line on a pregnancy test is still a positive result. The line darkness depends on your hCG concentration, which is still rising rapidly at this point. If you test again in two to three days, the line will likely be darker. What matters is that two lines appeared, not how bold they are.

Can I still exercise at 4 weeks pregnant?

In most cases, yes. If you were exercising before pregnancy, it's generally safe to continue your routine with some modifications. Avoid activities with a high risk of falling or abdominal impact, stay hydrated, and listen to your body. If you have any concerns, bring them up at your first prenatal visit.

Why do I feel like I have PMS?

Early pregnancy symptoms and premenstrual symptoms are driven by many of the same hormones, particularly progesterone. That's why the two can feel so similar — breast soreness, bloating, mood swings, and fatigue are common to both. The key difference is that your period won't arrive, and these feelings will continue to evolve over the coming weeks.

This Week's Tip

Schedule your first prenatal appointment. Even though most providers won't see you until around week 8, calling now is important. It gets you on the schedule, gives you a chance to ask questions, and helps you establish a relationship with your care team early. When you call, they'll likely ask about the first day of your last period, any medications you're taking, and your medical history. Write this information down ahead of time so you have it ready. If you don't have a provider yet, ask friends for recommendations, check your insurance network, or look into local midwifery practices — finding someone you trust and feel comfortable with makes a real difference.

Sources

• American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy.
• Mayo Clinic. Fetal Development: The 1st Trimester.

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