GUIDE
Published on ·Updated on
Your baby is the size of a sesame seed — and the heart is beginning to form.
Week 5 is when pregnancy starts to feel real in your body. hCG levels are surging, the embryo's tiny heart tube is taking shape and may begin its first fluttering beats by the end of this week, and the neural tube — the precursor to the brain and spinal cord — is forming. You may also be meeting morning sickness for the first time.
Be prepared from day one
“There's a host of reasons why prenatal care is important. The earlier that we can get patients seen, the earlier we can start interventions that can improve these longer-term outcomes.”
Your baby is about the size of a sesame seed — roughly 2 millimeters from head to tail. It does not look like a baby yet. The embryo at this stage resembles a tiny tadpole with a head end, a tail end, and a curved C-shape.
The most remarkable development this week is the heart. A simple tube-shaped structure is forming that will eventually become the four-chambered heart. By the end of week 5 or early week 6, this primitive heart tube may begin to beat — the first organ to function in the developing embryo. The heartbeat at this stage is rapid, around 100-120 beats per minute, and it will accelerate over the coming weeks.
The neural tube is also forming this week. This is a flat plate of cells that folds in on itself to create a tube running along the embryo's back. The top of the neural tube will become the brain; the bottom will become the spinal cord. This is why folic acid is so critical right now — adequate folate helps the neural tube close properly, reducing the risk of defects like spina bifida and anencephaly.
The three germ layers established last week — ectoderm, mesoderm, and endoderm — are now rapidly differentiating into specific structures. Buds that will become the arms and legs are just beginning to appear.
For a look at what happened last week, see our week 4 guide.
This is often the week that pregnancy symptoms announce themselves. hCG levels are doubling every 48-72 hours, and your body is responding to the hormonal surge.
Nausea is the symptom that gets the most attention — and for good reason. Morning sickness affects up to 80% of pregnant people, and it often begins around week 5. Despite its name, it can strike at any hour. The exact cause is not fully understood, but rising hCG and estrogen levels are thought to be the primary triggers. An enhanced sense of smell, which is also hormone-driven, can make nausea worse by making previously neutral scents unbearable.
Fatigue at this stage can be profound. Your body is building the placenta — an entirely new organ — and your metabolic rate has already increased. Progesterone's sedative effect compounds the exhaustion. Do not fight it. Rest when you can. This level of fatigue typically improves in the second trimester.
Your breasts may feel noticeably different — tender, swollen, heavy, or tingly. The areolas may begin to darken slightly. These changes are driven by hCG, estrogen, and progesterone preparing your breast tissue for eventual milk production, even though that is months away.
Every pregnancy is different. You may experience all, some, or none of these symptoms.
The priorities this week are managing early symptoms, continuing good nutrition, and setting up your prenatal care.
If morning sickness has arrived, try eating small, frequent meals rather than three large ones. Keep crackers by your bed to eat before getting up. Ginger (tea, candies, or supplements) has evidence supporting its anti-nausea effects. Stay hydrated — dehydration worsens nausea. If you cannot keep any food or fluids down, call your provider.
Most providers schedule the first prenatal visit between weeks 8 and 10. Call now to get on the calendar — popular practices book out. Be prepared to give your last menstrual period date, medical history, and medication list.
No alcohol, no smoking, no recreational drugs. Limit caffeine to 200 mg per day. Avoid hot tubs and saunas — core body temperatures above 101 degrees Fahrenheit in the first trimester are associated with an increased risk of neural tube defects.
In addition to your prenatal vitamin, eating foods naturally high in folate supports neural tube development. Good sources include spinach, lentils, black beans, asparagus, avocado, and fortified cereals.
When in doubt, call your provider. No question is too small during pregnancy.
Next week, your baby's heartbeat will likely be strong enough to detect on an ultrasound, and limb buds will be more defined. Nausea may intensify as hCG continues its steep climb.
Next up: Week 6. If nausea is already a problem, read our morning sickness guide and nausea remedies for evidence-based relief strategies. Your provider will likely order a dating ultrasound at your first appointment to confirm your due date. Also review our list of foods to avoid during pregnancy now that you have a confirmed positive.
This guide is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider with any questions about your pregnancy.
