How Pregnancy Weeks Are Counted
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. This means you are technically "2 weeks pregnant" before you even conceive.
This counting method exists because most women know their LMP date but not their exact ovulation/conception date. Conception typically occurs around day 14 of a 28-day cycle (week 2 of pregnancy). So when your doctor says you are "6 weeks pregnant," your embryo is actually about 4 weeks old. Full-term pregnancy is 40 weeks (280 days) from LMP, which is approximately 38 weeks from conception. The due date is calculated using Naegele's rule: LMP + 280 days. For cycles longer than 28 days, add the extra days. For shorter cycles, subtract. Only 4-5% of babies arrive on their exact due date (Mittendorf et al., 1990). Most births occur between 37 and 42 weeks, with 80% occurring within 2 weeks of the due date.
Baby Size by Week: The Complete Fruit Comparison
Doctors and parents love comparing baby size to fruits because it makes abstract measurements tangible.
| Week | Size Comparison | Length | Weight |
|---|
| 8 | Raspberry | 0.6 in | 0.04 oz |
| 12 | Lime | 2.1 in | 0.49 oz |
| 16 | Avocado | 4.6 in | 3.5 oz |
| 20 | Banana | 6.5 in | 10.2 oz |
| 24 | Corn | 11.8 in | 1.3 lbs |
| 28 | Eggplant | 14.8 in | 2.2 lbs |
| 32 | Squash | 16.7 in | 3.7 lbs |
| 36 | Honeydew | 18.7 in | 5.8 lbs |
| 40 | Watermelon | 20.2 in | 7.5 lbs |
These are averages. Individual babies vary significantly. The most rapid growth occurs in the third trimester: baby roughly doubles in weight between weeks 28 and 40. Length measurements switch from crown-to-rump (through week 20) to crown-to-heel (week 20 onward), which is why there appears to be a big jump at week 20. Use our Due Date Calculator for detailed due date estimation.
The Three Trimesters Explained
Each trimester brings distinct developmental milestones for baby and physical changes for mother.
First Trimester (Weeks 1-12): All major organs form. Heart starts beating at week 6. Morning sickness peaks at weeks 8-10. Highest miscarriage risk (80% occur in first trimester). Key tests: first ultrasound (8-12 weeks), NIPT screening (10+ weeks). Energy is often low due to rising progesterone.
Second Trimester (Weeks 13-27): Often called the "honeymoon trimester." Morning sickness typically resolves. Baby movement felt from 16-22 weeks (earlier in subsequent pregnancies). Anatomy scan at 18-22 weeks reveals gender and checks for structural abnormalities. Belly becomes visible. Energy returns for most women.
Third Trimester (Weeks 28-40): Rapid fetal growth. Lungs mature around week 34-36. Braxton Hicks contractions begin. Sleep becomes difficult. Baby turns head-down (usually by week 36). "Full term" starts at 39 weeks. Induction may be recommended after 41 weeks if labor has not started naturally.
Important Prenatal Tests by Week
Prenatal testing follows a specific timeline aligned with fetal development milestones.
Weeks 8-12: First ultrasound (confirms pregnancy, dates, heartbeat). Blood type, Rh factor, CBC, STI screening. Weeks 10-13: NIPT (non-invasive prenatal testing) for chromosomal conditions (Down syndrome, trisomy 18/13). 99% detection rate from a simple blood draw. Optional but increasingly standard. Weeks 11-14: Nuchal translucency ultrasound (measures fluid behind baby's neck). Weeks 15-20: Quad screen blood test. Amniocentesis if indicated (diagnostic, not screening). Weeks 18-22: Anatomy scan (detailed ultrasound checking all organs, limbs, spine, heart). Gender can be determined. Weeks 24-28: Glucose tolerance test for gestational diabetes. Rh antibody check if Rh-negative. Weeks 35-37: Group B Strep (GBS) swab. If positive, antibiotics during labor. Weekly from 36+: Non-stress test and cervical checks if indicated.
When to Call Your Doctor
Certain symptoms during pregnancy require immediate medical attention.
Call immediately for: Vaginal bleeding (especially with cramping), severe abdominal pain, sudden severe headache with vision changes (preeclampsia sign), water breaking before 37 weeks, significantly decreased fetal movement (fewer than 10 movements in 2 hours after 28 weeks), fever over 100.4F (38C), painful urination with fever (UTI/kidney infection risk). Call within 24 hours for: Persistent vomiting preventing hydration, swelling in face/hands (preeclampsia), painful or burning urination without fever, regular contractions before 37 weeks. Normal but uncomfortable: Round ligament pain (sharp side pain), Braxton Hicks (irregular, non-painful contractions), mild swelling in feet/ankles, heartburn, back pain. When in doubt, always call. No healthcare provider will think you are overreacting during pregnancy.
Medical Disclaimer: This calculator provides estimates based on Naegele's rule and average fetal development data. Every pregnancy is unique. The due date is an estimate, not a guarantee. Fetal size varies significantly between individuals. This tool is for informational purposes only and does not replace prenatal medical care. Always consult your OB-GYN or midwife for medical decisions. Sources: ACOG, WHO, Mittendorf et al. (1990), American Pregnancy Association.
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