Week 4
Your baby is the size of a poppy seed
What's happening this week
By week 4, implantation is complete and your baby — now officially called an embryo — is about the size of a poppy seed. But don't let the tiny size fool you. Inside that speck, the most fundamental architecture of a human being is taking shape.
The embryo has organized itself into three distinct layers, each destined to become different body systems. 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, pancreas, and digestive tract. Three layers, one future person.
The placenta is forming too. The trophoblast cells that burrowed into your uterine lining are now developing into a network of blood vessels that will eventually connect your bloodstream to your baby's, delivering oxygen and nutrients for the next eight months. The amniotic sac — the protective fluid-filled bubble your baby will float in — is also taking shape.
Perhaps most remarkably, the cells that will become your baby's heart are already beginning to organize. They won't start beating for another week or two, but the cardiac tissue is there, quietly assembling. From a poppy seed, a heartbeat is being built.
Your body is now producing measurable levels of hCG, which is why this is the week a pregnancy test can finally deliver that life-changing result.
Your body this week
This is the week you might miss your period — and for many people, that's the first clue that something is different. If your cycle is regular, a missed period is the most reliable early sign of pregnancy. But it's not the only one.
Rising hCG and progesterone levels may bring some early symptoms: breast tenderness or swelling, mild bloating, fatigue that hits out of nowhere, and possibly some mood changes. Some people describe it as feeling like their period is about to start — but it doesn't. Others feel completely normal.
Morning sickness usually doesn't start until around week 6, but a small number of people begin feeling queasy this early. If you do, it's likely due to the rapid rise in hCG.
This is a good time to take a home pregnancy test if you haven't already. Most tests are accurate from the first day of your missed period, though early-result tests may work a few days sooner. If you get a positive result, schedule your first prenatal appointment — most providers will see you between weeks 8 and 12. In the meantime, continue taking your prenatal vitamin, avoid alcohol and smoking, and limit caffeine to under 200mg per day (about one 12-ounce cup of coffee).
If you're on any medications, don't stop them without talking to your provider first — some medications are important to continue during pregnancy, and abruptly stopping can be harmful.
For dads
If your partner just showed you a positive pregnancy test, take a breath — this is real. The emotions might range from pure joy to quiet terror to 'wait, already?' and all of them are valid. There's no single correct reaction. What matters is what you do next: be present. Whether your partner is thrilled, nervous, or something in between, the most important thing right now is that you're in this together. Practically, here's what's coming: your partner will schedule a first prenatal appointment, usually around weeks 8-12. Between now and then, she should continue prenatal vitamins, avoid alcohol and raw fish, and limit caffeine. You can help by knowing these basics so she doesn't have to carry the mental load alone.
The early weeks can feel surreal — there's no bump, no ultrasound yet, no tangible proof except a line on a stick. Your partner might swing between excitement and anxiety, sometimes in the same hour. The best thing you can do right now is listen without trying to solve. If she says she's scared, don't say 'don't worry.' Say 'tell me what's on your mind.' Start reading about pregnancy too — not because she asks you to, but because it shows you're invested. And start thinking about when you want to share the news. Many couples wait until after the first trimester, but there's no rule — decide together what feels right for your family.
Common concerns
Is it normal to feel like my period is about to start?+
Yes — many people describe early pregnancy symptoms as feeling almost identical to PMS. Bloating, cramping, breast tenderness, and mood changes are common in both. The key difference is that your period doesn't arrive. This overlap happens because the same hormone (progesterone) drives both PMS symptoms and early pregnancy symptoms.
How accurate are home pregnancy tests?+
When used on or after the first day of your missed period, home pregnancy tests are about 99% accurate. Testing too early can produce a false negative because hCG levels may not yet be high enough to detect. If you get a negative result but still suspect you're pregnant, wait a few days and test again. A false positive is extremely rare.
Is it true that I need to stop drinking coffee?+
You don't need to quit caffeine entirely, but most healthcare providers recommend limiting intake to under 200mg per day — roughly one 12-ounce cup of brewed coffee. High caffeine intake has been associated with increased risk of miscarriage in some studies, though the evidence is mixed. If you're unsure, talk to your provider about what level is right for you.
When should I call a doctor about bleeding?+
Light spotting around the time of your expected period can be normal — it may be implantation bleeding or breakthrough bleeding from hormonal changes. However, contact your healthcare provider if you experience heavy bleeding (soaking a pad), severe cramping, or bleeding accompanied by dizziness or shoulder pain. These can be signs of complications that need evaluation.
Product picks for week 4
We may earn a small commission if you purchase through links on this page — at no extra cost to you. Learn more.
Home pregnancy test multi-pack
If you haven't tested yet, now's the time. Multi-packs let you confirm the result a day or two later for peace of mind.
Prenatal vitamins with DHA
Upgrade to a prenatal that includes DHA (omega-3) to support your baby's brain and eye development from the very start.
What to expect pregnancy book
A comprehensive week-by-week guide that answers the questions you didn't know you had. Great for both parents.
A quick note: This content is for informational purposes only and is not a substitute for professional medical advice. Always talk to your healthcare provider about any questions or concerns. Learn how we create our content.
Content based on guidance from the American College of Obstetricians and Gynecologists (ACOG), the American Academy of Pediatrics (AAP), Mayo Clinic, and peer-reviewed medical literature. Learn more about how we create our content.