Conception Calculator — Week 1 From Conception (Fetal Age)
You are 1 week from conception, which corresponds to 3 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 1
At 1 weeks from conception (3w gestational): Technically not yet conceived — this is the first week of your last menstrual period (LMP). Ovulation is expected around day 14.
Mother's body — what changes
1 weeks post-conception, your body is: You are menstruating. Body is shedding the previous cycle's endometrial lining and preparing follicles for the next ovulation.
Nutrition & supplements
Focus on folic acid 400–600 mcg/day (neural tube), iodine 150 mcg, and small frequent meals to counter nausea. No extra calories yet — normal maintenance intake is adequate. Week 1 from conception (3 weeks gestational) needs targeted micronutrients — the developmental window differs from LMP-based advice.
Safe exercise
150 minutes/week of moderate activity — brisk walking, stationary cycling, prenatal yoga. Stop any exercise that leaves you unable to hold a conversation. At 1 weeks from conception, low-impact activity supports placental blood flow without stressing the fresh implantation.
Doctor visit checklist — week 1
- ✓Start prenatal folate 400 mcg/day if trying to conceive
- ✓Note: 1 weeks from conception = 3 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 loss of pregnancy symptoms + spotting
- ⚠Severe one-sided pain (ectopic pregnancy signal)
Week 1 — Frequently asked questions
How accurate is the Conception Calculator at week 1?
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 1?
Approximately the size of a poppy seed — length 0 cm and weight 0 g. These are median values from ACOG dating criteria and Mayo Clinic conception timeline references.
Which symptoms are normal in week 1?
You are menstruating. Body is shedding the previous cycle's endometrial lining and preparing follicles for the next ovulation. Every pregnancy is different — call your provider if anything on the warning-signs list appears.
Should I have an ultrasound at week 1?
Start prenatal folate 400 mcg/day if trying to conceive. Routine scan windows are: dating (10–13w), anatomy (18–22w), growth (28–32w).
How much weight should I gain by week 1?
Approximately 0.1 kg cumulative if you started at a normal BMI (IOM guideline). Underweight mothers gain more; overweight mothers less.
1 weeks from conception is how many weeks pregnant?
1 weeks from conception = 3 weeks gestational (LMP-based). All clinical care uses LMP dating unless IVF or dating scan overrides.
Can I feel conception happening at week 1?
Some women report mild pelvic twinges (mittelschmerz), spotting from implantation, or breast tenderness. Most feel nothing until missed period.
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.