First Trimester Week by Week — What to Expect (Weeks 1-13)

Weeks 1 and 2 are counted from the first day of your last menstrual period, so you aren't technically pregnant yet — your body is preparing for ovulation and potential fertilization. Conception typically happens around week 2-3 when an egg is fertilized, and by the end of week 3 the fertilized egg has traveled down the fallopian tube and begun implanting in the uterine lining.

By week 4, the embryo is about the size of a poppy seed and is made up of two layers of cells that will develop into all the organs and body parts. The amniotic sac and yolk sac are forming, and the placenta is beginning to develop. Your body starts producing hCG (human chorionic gonadotropin), which is the hormone that pregnancy tests detect.

At this stage, most women don't know they're pregnant yet. Some may notice very light spotting (implantation bleeding), mild cramping, breast tenderness, or fatigue. These early symptoms can easily be mistaken for PMS. A home pregnancy test should be able to detect pregnancy around the time of your missed period — roughly the end of week 4. If your test is positive, call your provider to schedule your first prenatal appointment, which usually happens between weeks 8-10.

Weeks 5-7 are a period of rapid and critical development. By week 5, the embryo is about the size of a sesame seed and the neural tube (which becomes the brain and spinal cord) is forming. The heart begins to beat — usually detectable by transvaginal ultrasound around week 6 at roughly 100-120 beats per minute. Tiny buds that will become arms and legs are emerging.

By week 7, the embryo is about the size of a blueberry. Facial features are starting to form — tiny nostrils, eye lenses, and the beginnings of ears. The brain is growing rapidly, generating about 100 new brain cells every minute. Internal organs like the liver, kidneys, and lungs are in their earliest stages of development.

This is when most women start feeling unmistakably pregnant. Morning sickness (which can strike any time of day) affects 70-80% of women and typically begins around week 6. You may also experience extreme fatigue due to rising progesterone levels, frequent urination as your uterus starts to press on the bladder, heightened sense of smell, food aversions or cravings, breast soreness and swelling, and mood swings. These symptoms, while uncomfortable, are actually reassuring signs that pregnancy hormones are at healthy levels. Taking prenatal vitamins with folate (at least 400mcg daily) is especially critical during this period, as the neural tube closes between weeks 6-7.

By week 8, the embryo officially becomes a fetus. It's about the size of a raspberry, and all essential organs have begun forming. Fingers and toes are starting to develop (though still webbed), and the upper lip, nose, and eyelids are taking shape. The fetus is making spontaneous movements, though it's far too small for you to feel them.

Week 9 brings more definition — the tail-like structure at the base of the spinal cord has disappeared, and the fetus looks more recognizably human. Muscles are forming, and tiny bones are beginning to harden (ossify). By week 10, the fetus is about the size of a strawberry, and all vital organs are in place and beginning to function. The brain is producing nearly 250,000 new neurons every minute.

This is typically when you'll have your first prenatal appointment. Expect a thorough health history, blood work (blood type, Rh factor, CBC, STI screening, immunity checks), urine tests, blood pressure measurement, and possibly your first ultrasound. Hearing the heartbeat for the first time — now beating at a rapid 170-180 bpm — is often an emotional milestone. Your provider will also discuss genetic screening options, including the first-trimester screening (nuchal translucency ultrasound combined with blood work) that's done between weeks 11-14.

By week 11, the fetus is about the size of a fig and is busy developing tooth buds, nail beds, and hair follicles. External genitalia are beginning to differentiate, though it's usually too early to determine sex on ultrasound. The fetus can open and close its fists, and the diaphragm is forming, which means hiccups may start (you won't feel them yet).

At week 12, the fetus is about the size of a lime. The kidneys are starting to produce urine, and the digestive system is practicing contraction movements. The placenta has taken over hormone production from the ovary, which is why many women start feeling relief from nausea around this time. The risk of miscarriage drops significantly after a heartbeat is confirmed at 12 weeks — to about 2-3%.

Week 13 marks the end of the first trimester. The fetus is about the size of a lemon, roughly 3 inches long, and has fingerprints forming, vocal cords developing, and intestines moving from the umbilical cord into the abdomen. If you opted for first-trimester screening, results typically come back around now.

For many women, the end of the first trimester brings welcome relief — energy levels often improve, nausea typically begins to fade, and the risk of pregnancy loss is now much lower. You may notice a small baby bump beginning to show, though first-time mothers often don't show until closer to weeks 16-20.

Morning sickness affects up to 80% of pregnant women and typically peaks between weeks 8-11. For most, it's manageable with lifestyle changes, but about 2-3% of women develop hyperemesis gravidarum — severe nausea and vomiting that can lead to dehydration and weight loss requiring medical treatment.

Evidence-based strategies for managing typical morning sickness include eating small, frequent meals (every 2-3 hours) rather than three large ones, keeping plain crackers or dry toast by your bedside to eat before getting up, staying hydrated with small sips throughout the day (ginger ale, electrolyte drinks, or ice chips if you can't keep water down), avoiding strong smells and cooking odors (cold foods tend to have less smell), and trying vitamin B6 (pyridoxine) — 25mg three times daily has been shown in studies to reduce nausea.

Ginger has solid evidence behind it — studies show that 250mg of ginger root capsules four times daily significantly reduces nausea. Acupressure wristbands (Sea-Bands) placed on the P6 point on your inner wrist may also help some women.

If lifestyle measures aren't enough, talk to your provider about Doxylamine-B6 (Unisom + vitamin B6), which is the first-line medication for pregnancy nausea and has an excellent safety record. For severe cases, prescription antiemetics like ondansetron may be discussed.

Call your provider if you're vomiting more than three times a day, can't keep any food or liquid down for 24 hours, are losing weight, feel dizzy or faint, or have dark-colored urine (a sign of dehydration).

First-trimester screening is offered between weeks 11-14 and is designed to assess the risk of chromosomal conditions — primarily Down syndrome (trisomy 21), trisomy 18, and trisomy 13. This screening is optional but recommended, and it's important to understand that it's a risk assessment, not a diagnosis.

The standard first-trimester screening combines two components: a nuchal translucency (NT) ultrasound, which measures the fluid-filled space at the back of the baby's neck (a thicker measurement can indicate higher risk), and a maternal blood test that measures two proteins — PAPP-A and free beta-hCG. These results, combined with your age, give you a risk ratio.

Cell-free DNA screening (also called NIPT or non-invasive prenatal testing) is another option available as early as week 10. It analyzes fragments of placental DNA circulating in your blood and is more accurate than traditional screening for common trisomies — with a detection rate above 99% for Down syndrome. It can also reveal the baby's sex and screen for sex chromosome conditions.

If any screening comes back high-risk, your provider will offer diagnostic testing — chorionic villus sampling (CVS), performed between weeks 10-13, which takes a tiny sample of placental tissue, or amniocentesis, done later at weeks 15-20. These diagnostic tests are definitive (99%+ accuracy) but carry a small miscarriage risk of about 0.1-0.3%.

No screening is mandatory. Your provider or a genetic counselor can help you decide what level of information is right for you based on your medical history, age, and personal preferences.