First Trimester Guide: What Happens in Weeks 4-13

At 5 weeks, most women don't look pregnant at all — but a heartbeat is already forming. By week 12, the fetus has all its major organs, fingernails, and can make facial expressions. The first trimester is the most dramatic 10 weeks of development in a human life, and also the period with the most uncertainty. Miscarriage risk is highest in these weeks, prenatal testing windows are narrow, and the physical symptoms — nausea, exhaustion, breast tenderness — often hit hardest exactly when most women aren't ready to tell anyone what's happening.

Understanding the timeline helps. Knowing what's developing when, which appointments need to happen in which windows, and what the test results actually mean makes the first trimester less anxious and more navigable.

Weeks 4-6: The Implantation Phase

A positive pregnancy test in week 4 detects the hormone hCG (human chorionic gonadotropin), which the embryo begins producing after implantation in the uterine wall. At this stage, the embryo is about 1-2mm — smaller than a grain of rice.

By week 5, the embryo has a primitive heart tube. It isn't a four-chamber heart yet, but electrical activity begins, and this is what shows up as a heartbeat on an early ultrasound. The heart rate at week 6 is typically 90-110 beats per minute and will accelerate to 160-180 bpm by weeks 9-10 before settling into the 120-160 range for the remainder of pregnancy.

Symptoms during weeks 4-6: The most common first trimester symptoms begin in this window. Breast tenderness and fatigue often appear before morning sickness. Nausea typically starts around week 6, triggered by rising hCG levels. For roughly 75% of pregnant women, nausea is present to some degree; for about 30%, it involves vomiting.

First scan: A transvaginal ultrasound at 6-7 weeks can confirm a heartbeat and rule out ectopic pregnancy (embryo implanted outside the uterus). This scan is not always routine — your OB or midwife will schedule it if there's any reason for concern or if you had fertility treatment.

Weeks 7-9: Organ Formation

By week 8, all major organ systems have begun forming. This is the embryonic period — the time when the embryo is most sensitive to environmental factors, including medications, alcohol, and certain infections. The arms and leg buds appear. Facial features begin to differentiate. The neural tube — which will become the brain and spinal cord — closes during weeks 3-4; by week 8, the brain has distinct regions.

At week 8, the embryo is approximately 1.6cm (0.6 inches). By week 9, it's about 2.3cm (0.9 inches).

The first OB appointment is typically scheduled for weeks 8-10. At this appointment, your provider will:

• Confirm the pregnancy and gestational age via ultrasound
• Establish your expected due date
• Order first trimester bloodwork (blood type, Rh factor, CBC, STI screening, thyroid function)
• Review your medications and supplements
• Discuss prenatal vitamins and folic acid if you haven't started

Why exact week matters: The nuchal translucency scan (discussed below) has a specific window of weeks 11-13 weeks and 6 days. Missing that window means losing access to first-trimester Down syndrome screening. Knowing your exact weeks ensures you schedule correctly.

Weeks 10-13: End of the Embryonic Period

At week 10, the embryo officially becomes a fetus. The most critical period of organ formation is complete. Miscarriage risk drops significantly after week 10 — from roughly 10-15% in weeks 6-9 to approximately 1-5% after week 10 for confirmed viable pregnancies.

By week 12, the fetus is approximately 6cm (2.4 inches) and weighs about 14g. Fingernails and toenails are forming. The fetus can make facial expressions, open and close its hands, and respond to stimulation. If you have an ultrasound at 12-13 weeks, you'll see active movement even if you can't feel it yet.

Nuchal translucency (NT) scan: This first-trimester screening occurs between weeks 11 and 13 weeks 6 days — no earlier, no later. The ultrasound measures the fluid at the back of the fetal neck. Combined with first-trimester blood markers (PAPP-A and free beta-hCG), it provides a risk assessment for chromosomal abnormalities including Down syndrome (trisomy 21), trisomy 18, and trisomy 13.

The NT scan is screening, not diagnosis. An elevated NT measurement means increased risk, not confirmed diagnosis. Further testing — cell-free fetal DNA (cfDNA/NIPT) or diagnostic tests like CVS — may be offered based on results.

NIPT (Non-Invasive Prenatal Testing): A blood test for the pregnant person, available from week 10 onward, that analyzes fetal DNA fragments in the maternal bloodstream. NIPT screens for the major chromosomal conditions with high sensitivity and specificity. Some providers offer it as routine screening; others offer it selectively. Results typically return in 7-14 days.

First Trimester Weight Gain Expectations

The Institute of Medicine (IOM) recommends total first-trimester weight gain of 1-4.5 lbs for women with a normal pre-pregnancy BMI (18.5-24.9). For context:

• Underweight (BMI < 18.5): 1-4.5 lbs in first trimester, 28-40 lbs total
• Normal weight (BMI 18.5-24.9): 1-4.5 lbs in first trimester, 25-35 lbs total
• Overweight (BMI 25-29.9): 0.5-2 lbs in first trimester, 15-25 lbs total
• Obese (BMI 30+): 0.5-2 lbs in first trimester, 11-20 lbs total

Many women gain very little or even lose weight in the first trimester due to nausea and food aversions. This is typically not a concern if you're maintaining hydration and eating what you can. The bulk of pregnancy weight gain occurs in the second and third trimesters.

The Pregnancy Weight Gain Calculator tracks your gain against IOM guidelines by trimester, so you can see whether you're within the recommended range at each checkup.

Using the Pregnancy Week Calculator

Knowing your exact gestational week is more important in the first trimester than at any other time. The NT scan window is only 17 days wide. NIPT eligibility starts at exactly 10 weeks. First-trimester bloodwork is most informative when combined with an accurate gestational age.

The Pregnancy Week Calculator takes your last menstrual period (LMP) date and calculates your current week and day, your trimester, your estimated due date (EDD), and the dates of upcoming milestone windows. It also shows you how far away the NT scan window is and when you'd enter the second trimester.

If you don't know your LMP or your cycles are irregular, your OB can establish gestational age via ultrasound measurements — crown-rump length (CRL) in the first trimester is highly accurate for dating.

This guide is for informational purposes only. Always consult your OB, midwife, or healthcare provider for guidance specific to your pregnancy.