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Pregnancy Trimesters

Dating from Last Menstrual Period (LMP)

LMP dating is the standard method healthcare providers use because most women remember their last period date. Pregnancy is calculated from the first day of the LMP, even though conception doesn't occur until approximately 2 weeks later. This means when you're "4 weeks pregnant," conception occurred only about 2 weeks ago. The 40-week pregnancy (280 days) is counted from LMP, not from conception. Naegele's Rule calculates due date as: LMP + 280 days, or LMP + 7 days - 3 months + 1 year.

Dating from Conception Date

If conception date is known (through fertility tracking, IVF, or single intercourse instance), pregnancy can be dated more precisely. From conception, add 266 days (38 weeks) to calculate due date. This method is more accurate than LMP for women with irregular cycles or those using assisted reproductive technology. When conception date is known, healthcare providers may adjust LMP-based calculations. However, even with known conception date, natural variation means only 5% of babies arrive exactly on due date.

Dating from Ultrasound

Ultrasound dating is most accurate when performed in the first trimester (8-13 weeks). Early ultrasounds measure crown-rump length (CRL) to estimate gestational age within ±5-7 days accuracy. First-trimester ultrasounds are more accurate than LMP dating for women with irregular cycles, uncertain LMP, or discrepancies between LMP and physical exam. If ultrasound dating differs from LMP dating by >7 days, healthcare providers typically use the ultrasound date. Later ultrasounds (second/third trimester) are less accurate for dating due to individual growth variations.

Why Due Dates Are Estimates

Only 5% of babies arrive on their exact due date. Normal pregnancy lasts 37-42 weeks—a full 5-week range. About 70% of babies are born within 10 days of the due date (before or after). Factors affecting timing include first vs. subsequent pregnancies (first pregnancies average 8 days longer), maternal age, genetics, and individual cycle variations. "Full term" is now defined as 39-40 weeks, with "early term" at 37-38 weeks and "late term" at 41-42 weeks. Pregnancies beyond 42 weeks (post-term) typically require medical intervention.

First Trimester (Weeks 1-12)

The first trimester is the period of most rapid fetal development. All major organs and body systems begin forming—brain, spinal cord, heart, arms, legs, fingers, toes. The heart starts beating around week 5-6, detectable by ultrasound. By week 12, the fetus is fully formed at about 2-3 inches long, though still developing. Morning sickness, fatigue, breast tenderness, and frequent urination are common symptoms due to rapidly rising hormone levels (hCG, progesterone, estrogen). Miscarriage risk is highest in first trimester (10-20% of known pregnancies), decreasing significantly after week 12.

Second Trimester (Weeks 13-27)

The second trimester is often called the "honeymoon period" because morning sickness typically subsides and energy returns. The fetus grows rapidly from about 3 inches to 14 inches and develops distinct features—facial features, fingerprints, hair. Most women feel fetal movement (quickening) around weeks 16-25, earlier in subsequent pregnancies. The 20-week anatomy scan ultrasound checks fetal development and can reveal biological sex. Belly growth becomes obvious as the uterus expands above the pelvis. Common symptoms include round ligament pain, backache, skin changes (linea nigra, darkened nipples), and pregnancy "glow" from increased blood flow.

Third Trimester (Weeks 28-40+)

The third trimester involves final fetal development and rapid weight gain—baby gains about half its birth weight in the last 6-8 weeks. Lungs mature (surfactant production begins around week 26, critical for breathing), brain develops rapidly, and baby assumes head-down position for birth (usually by week 36). Common symptoms include shortness of breath (uterus pressing diaphragm), frequent urination (bladder pressure), heartburn, swelling (edema), Braxton Hicks contractions (practice contractions), and difficulty sleeping. Regular prenatal visits monitor for complications like preeclampsia, gestational diabetes, and preterm labor. Labor typically begins spontaneously between weeks 37-42.

Common Early Pregnancy Symptoms

Early pregnancy symptoms vary widely—some women notice changes within days of conception, while others have no symptoms for weeks. Common signs include: Missed period (most obvious indicator); Implantation bleeding (light spotting 6-12 days post-conception); Breast tenderness and swelling; Fatigue and exhaustion; Nausea with or without vomiting (morning sickness, though can occur any time); Frequent urination; Food aversions or cravings; Mood swings; Light cramping; Basal body temperature remaining elevated. However, many early pregnancy symptoms mimic PMS, making them unreliable indicators without a positive pregnancy test.

When to Take a Pregnancy Test

Home pregnancy tests detect hCG hormone in urine, which doubles every 48-72 hours in early pregnancy. For most accurate results, test on the first day of your missed period or later (approximately 14 days post-ovulation/conception). Testing earlier may produce false negatives because hCG levels are still too low. "Early detection" tests claim to detect pregnancy up to 6 days before missed period, but accuracy is only 50-70% before missed period. First morning urine has highest hCG concentration. Blood tests performed by healthcare providers can detect pregnancy earlier (7-12 days post-conception) and provide quantitative hCG levels.

Implantation Bleeding vs. Period

Implantation bleeding occurs in about 15-25% of pregnancies when the embryo burrows into the uterine lining, typically 6-12 days after conception. It's usually light pink or brown spotting (not red like period blood), lasts hours to 2 days (not 3-7 days like periods), and involves minimal flow (not enough to fill pads/tampons). Some women experience mild cramping. Implantation bleeding occurs before expected period—if spotting happens on or after the day your period is due, it's more likely early period or other cause rather than implantation. Heavy bleeding, severe cramping, or bleeding after positive pregnancy test warrant medical evaluation.

First Prenatal Appointment

First prenatal visit typically occurs at 8-12 weeks after LMP, once pregnancy is confirmed with home test. Earlier visits may be scheduled for high-risk pregnancies or specific concerns. The appointment includes: Confirmation of pregnancy (urine or blood test); Medical history review; Physical exam and pelvic exam; Calculation of due date; Dating ultrasound (if timing is uncertain or concerning symptoms); Blood tests (blood type, Rh factor, anemia, infectious diseases, immunity); Discussion of prenatal vitamins (folic acid crucial for neural tube development); and Lifestyle guidance (nutrition, exercise, medications to avoid, prenatal care schedule). Establish prenatal care early to monitor maternal and fetal health throughout pregnancy.