Conception Calculator — Week 21 From Conception (Fetal Age)
You are 21 weeks from conception, which corresponds to 23 weeks of gestational age (LMP-based). This page focuses on true fetal age — how long the baby has actually been developing since fertilisation.
Conception Calculator
Enter your LMP. Fetal age = gestational age − 2 weeks.
Baby at week 21
At 21 weeks from conception (23w gestational): Rapid brain growth. Digestive system produces meconium. Fetus responds to sound with movement.
Mother's body — what changes
21 weeks post-conception, your body is: Varicose veins may appear. Heartburn common as uterus pushes stomach up.
Nutrition & supplements
Add ~340 kcal/day above pre-pregnancy baseline. Prioritise iron (27 mg/day), calcium (1000 mg), omega-3 DHA (200–300 mg) and 71 g protein. Continue prenatal vitamin. Week 21 from conception (23 weeks gestational) needs targeted micronutrients — the developmental window differs from LMP-based advice.
Safe exercise
Continue 150 min/week. Avoid supine (flat-on-back) exercises after week 20 — they compress the vena cava. Swimming and prenatal Pilates are ideal now. At 21 weeks from conception, low-impact activity supports placental blood flow without stressing the fresh implantation.
Doctor visit checklist — week 21
- ✓Start iron supplement if hemoglobin <11 g/dL
- ✓Note: 21 weeks from conception = 23 weeks LMP — clinic dating uses LMP
- ✓Confirm dating with crown-rump length at 8–13 weeks (±5 days accuracy)
Warning signs — call your provider immediately
- ⚠Heavy bleeding (soaking a pad in an hour)
- ⚠Severe abdominal pain that does not ease
- ⚠Fever above 38°C / 100.4°F
- ⚠Sudden gush of fluid (early rupture of membranes)
- ⚠Absent fetal movement for 12+ hours after 24 weeks
- ⚠Severe headache + visual disturbance (preeclampsia)
Week 21 — Frequently asked questions
How accurate is the Conception Calculator at week 21?
The calculator uses standard gestational-age arithmetic (LMP + 280 days for standard; egg retrieval + 266 days for IVF day-5). Ultrasound crown-rump length between weeks 8–13 is more accurate (±5 days) than LMP — adjust EDD accordingly if the scan differs by more than 7 days.
What size is my baby at week 21?
Approximately the size of a carrot — length 26.7 cm and weight 360 g. These are median values from ACOG dating criteria and Mayo Clinic conception timeline references.
Which symptoms are normal in week 21?
Varicose veins may appear. Heartburn common as uterus pushes stomach up. Every pregnancy is different — call your provider if anything on the warning-signs list appears.
Should I have an ultrasound at week 21?
Start iron supplement if hemoglobin <11 g/dL. Routine scan windows are: dating (10–13w), anatomy (18–22w), growth (28–32w).
How much weight should I gain by week 21?
Approximately 5.5 kg cumulative if you started at a normal BMI (IOM guideline). Underweight mothers gain more; overweight mothers less.
21 weeks from conception is how many weeks pregnant?
21 weeks from conception = 23 weeks gestational (LMP-based). All clinical care uses LMP dating unless IVF or dating scan overrides.
Can I feel conception happening at week 21?
No — conception itself is not felt. Later pregnancy symptoms are hormone-driven, not conception-related.
Content aligned with ACOG dating criteria and Mayo Clinic conception timeline. Reviewed by Dr. Maria Rehman, MD (OB/GYN). This page is educational and does not replace personal medical advice — call your obstetrician for any concern.