Week 5 of pregnancy is when the embryo transitions from a cluster of cells into a recognizable structure with the beginnings of a brain, spinal cord, and heart. It’s still tiny, but the pace of development is remarkable. Most of what happens this week takes place at the microscopic level, yet these changes lay the foundation for every major organ system.
How the Embryo Is Developing
At five weeks of gestation (about three weeks after fertilization), the embryo is roughly the size of a small seed. It’s nestled inside a fluid-filled gestational sac in the uterus, drawing nutrients not from a placenta, which hasn’t fully formed yet, but from a structure called the yolk sac. Despite its name, the human yolk sac contains no actual yolk. Instead, it handles several jobs that are essential at this stage: producing the embryo’s first blood cells, supplying oxygen, and exchanging nutrients between the mother’s body and the embryo through an extensive network of tiny blood vessels. This system, sometimes called vitelline circulation, keeps the embryo alive until the placenta takes over later in the first trimester.
The Neural Tube Starts to Close
One of the most critical events of week 5 is neural tube formation. The neural tube is the precursor to the brain and spinal cord. A flat strip of specialized cells called the neural plate appears around this time, then folds inward and begins closing into a tube. By the end of the sixth gestational week (about 28 days after fertilization), the tube is typically sealed completely. Failures in this closure process are irreversible and can lead to conditions like spina bifida, where part of the spinal cord doesn’t close properly, or anencephaly, a severe brain defect.
This is exactly why folic acid matters so much in early pregnancy. The CDC recommends 400 micrograms daily for all women who could become pregnant, ideally starting before conception. By the time most people realize they’re pregnant, the neural tube is already forming or has finished closing. If you’re newly finding out at week 5, you’re right in the window where adequate folic acid intake makes the biggest difference.
The Heart Begins to Beat
The embryo’s cardiovascular system is the first organ system to become functional, and it starts taking shape during week 5. Initially, the heart is just a short, straight tube running along the front of the embryo. Over the course of several days, this tube lengthens and curves into a C-shape, which is the beginning of the looping process that will eventually create the four-chambered heart.
The first contractions of the immature heart cells appear as small, irregular twitches within localized areas of the developing muscle. These aren’t the rhythmic, coordinated beats you’d associate with a functioning heart. They’re more like spontaneous electrical impulses firing in patches of tissue. By the end of week 5, though, the heart tube may be pulsing around 110 times per minute and establishing the first unidirectional flow of plasma through the embryo’s body. Research on the timing of these first heartbeats shows they can appear anywhere from 20 to 35 days after fertilization, which means week 5 falls right in the middle of that range for many embryos.
What an Ultrasound Can Show
If you have a transvaginal ultrasound at five weeks, don’t expect to see much. At this point, the ultrasound typically shows a small fluid collection within the uterine lining representing the gestational sac. By about five and a half weeks, a tiny bubble-like structure measuring 3 to 5 millimeters may appear inside the sac. That’s the yolk sac.
You won’t see an embryo with recognizable features, and a heartbeat usually isn’t detectable until around week 6. This is why many providers prefer to wait until at least six or seven weeks for a first ultrasound. An earlier scan can cause unnecessary anxiety if it doesn’t show what you’re expecting. A transvaginal approach (rather than the wand-on-the-belly type) is important at this stage because it produces much higher-quality images of structures this small.
What You Might Be Feeling
Many women feel nothing at all during week 5. Others start noticing the first hints of pregnancy symptoms, which can include breast tenderness, bloating, mood swings, and fatigue. These are driven largely by rising levels of hCG, the hormone your body produces after implantation. At five weeks, hCG levels typically range from 200 to 7,000 units per liter, a wide range that reflects how much variation is normal from one pregnancy to the next.
Nausea may or may not have started. For many people, morning sickness doesn’t kick in until closer to week 6 or 7. If you’re already feeling nauseous, that’s within the normal range too. The intensity of your symptoms has no reliable connection to the health of your pregnancy.
Warning Signs to Know About
Week 5 is also the time frame when an ectopic pregnancy, where the fertilized egg implants outside the uterus (usually in a fallopian tube), can start producing symptoms. You may not notice anything unusual at first because early ectopic pregnancies produce the same signs as a normal pregnancy: a missed period, breast tenderness, nausea, and a positive pregnancy test.
The first warning signs are often light vaginal bleeding paired with pelvic pain. If blood leaks from the fallopian tube, you might feel shoulder pain or pressure in your lower pelvis. A growing ectopic pregnancy can eventually rupture the tube, causing heavy internal bleeding with symptoms like extreme lightheadedness, fainting, and shock. Severe abdominal pain with vaginal bleeding, sudden dizziness, or shoulder pain all warrant emergency medical attention.
Some light spotting on its own is common in early pregnancy and doesn’t necessarily signal a problem. But when spotting comes with one-sided pelvic pain or any of the other symptoms above, it’s worth getting checked quickly rather than waiting for your next scheduled appointment.