Week 21 of Pregnancy
Learn about your baby's development and what to expect at 21 weeks pregnant.
Baby's Size This Week
Baby Development at Week 21
- ● Your baby is approximately 26.7 cm long and weighs around 360g — roughly the size of a large carrot.
- ● The lungs are developing rapidly, with the airways branching and the cells beginning to produce surfactant, which will be essential for breathing after birth.
- ● Your baby has distinct sleep and wake cycles and is becoming more active with purposeful kicks, rolls, and punches.
- ● The liver and spleen are actively producing blood cells, while the bone marrow is maturing to eventually take over this role.
- ● Taste buds on the tongue are fully developed, and your baby is tasting the flavors of your diet through swallowed amniotic fluid.
- ● The eyebrows and eyelashes are fully formed, and your baby's eyes are structurally complete, though the iris still lacks pigment.
Your Body at Week 21
- ● Your growing belly may cause backaches and difficulty finding a comfortable sleeping position — a pregnancy pillow is highly recommended.
- ● Braxton Hicks contractions (mild, irregular practice contractions) may begin this trimester — they are painless and irregular, unlike true labor.
- ● Heartburn and indigestion are common as the growing uterus compresses the stomach and progesterone relaxes the esophageal valve.
- ● Leg cramps, especially at night, may intensify — ensuring adequate calcium and magnesium intake can help.
- ● Increased vaginal discharge (leukorrhea) is normal and may become heavier as the pregnancy progresses.
- ● Nasal congestion, shortness of breath, and feeling warmer than usual continue as blood volume and cardiac output increase.
Week 21 Checklist
- ☐ Begin formal movement tracking — note when your baby is most active each day and watch for consistent patterns.
- ☐ Research and register for birthing classes, newborn care classes, and breastfeeding preparation courses.
- ☐ Discuss your birth plan ideas with your provider — including pain management preferences, labor positions, and postpartum wishes.
- ☐ Focus on adequate iron and protein intake to support fetal growth and prevent iron-deficiency anemia.
- ☐ Start looking into postpartum support: hiring a doula, postpartum nurse, or identifying family support for after the birth.
- ☐ Schedule your glucose challenge test (GCT) for gestational diabetes, typically done between weeks 24-28.
Frequently Asked Questions
What should I expect at 21 weeks pregnant?
At 21 weeks, you are past the halfway point with a baby about 360g who is tasting your food, sleeping and waking on a schedule, and kicking with increasing force. Most anatomy scan results are in, and your care transitions to monitoring growth and preparing for the third trimester. Common symptoms include back pain, heartburn, Braxton Hicks, and strong fetal movement.
What are Braxton Hicks contractions and are they dangerous?
Braxton Hicks contractions are irregular, painless practice contractions of the uterus that can begin as early as the second trimester. They feel like a tightening or hardening of the abdomen that lasts 30 seconds to 2 minutes, then releases. They are not dangerous and do not dilate the cervix. They are triggered by dehydration, physical activity, or a full bladder. Drink water and rest when they occur. Contact your provider if contractions become regular, painful, or occur more than 4 per hour before 37 weeks.
My baby seems to move most at night — is that normal?
Yes, many babies are most active in the evening and at night. This may be because daytime movement (your walking) lulls them to sleep, while the relative stillness of night allows them to become more active. You may also simply be more aware of movement when you are lying quietly. This nocturnal pattern is completely normal and often continues after birth for a while.
What is surfactant and why does it matter for my baby's lungs?
Surfactant is a substance produced by specialized cells in the lungs (type II pneumocytes) that prevents the tiny air sacs (alveoli) from collapsing after each breath. Without adequate surfactant, lungs collapse with every exhalation, causing respiratory distress syndrome (RDS) — the primary breathing problem in premature babies. Surfactant production begins around week 20 but reaches adequate levels for survival outside the womb only around weeks 34-36, which is why very preterm birth carries respiratory risks.
What is gestational diabetes and when is it tested?
Gestational diabetes (GDM) is a form of diabetes that develops during pregnancy when the placental hormones interfere with insulin function, causing high blood sugar. It affects about 6-9% of pregnancies. The glucose challenge test (GCT) is a one-hour screening test done between weeks 24-28. If results are elevated, a 3-hour glucose tolerance test (GTT) is done to confirm the diagnosis. GDM managed with diet, exercise, and sometimes medication carries minimal risk to baby; unmanaged GDM can cause macrosomia (large baby), birth complications, and long-term metabolic risks for both mother and child.
Track Your Exact Pregnancy Week
Use our free Pregnancy Week Tracker to find out exactly how far along you are based on your last menstrual period or due date.
Track My Pregnancy →Sources
- ACOG
- WHO Fetal Growth Charts
- Mayo Clinic
Medical disclaimer: This content is for informational purposes only and is not intended as medical advice. Always consult a qualified healthcare provider for guidance specific to your pregnancy.