What are the dates behind a pregnancy?
Pregnancy and fertility involve surprisingly few concepts, all of which can be expressed as dates and counts. Once you know the first day of the last menstrual period (LMP), the rest is arithmetic: the due date is roughly LMP + 280 days (Naegele's rule), ovulation occurs about 14 days before the next period (regardless of cycle length), the fertile window is the 5 days before plus the day of ovulation, conception is roughly 266 days before the due date, and a baby's weight grows along well-studied curves that the ultrasound can compare to a reference. This page gives each its own calculator, all of which assume a "standard" 28-day cycle unless overridden. None of the estimates is exact: only ~4% of babies are born on their due date, ovulation can shift by several days month-to-month, and fetal weight estimates have ±10–15% error versus birth weight. The numbers are useful for planning, not promising.
Three vocab distinctions to keep straight: Gestational age (GA) is measured from the first day of the LMP, not from conception — a 6-week "pregnancy" has only had an embryo for about 4 weeks. Trimester 1 ends at week 13, Trimester 2 ends at week 27, and Trimester 3 ends at week 40. Fertile window (5–6 days) is much longer than the 12–24-hour window during which the egg is actually fertilizable because sperm can survive in the reproductive tract for up to 5 days. Due date (EDD) is the 280-day point from LMP, not a delivery-day prediction.
Due date — Naegele's rule
Naegele's rule (Franz Karl Naegele, 1812): EDD = LMP + 280 days (40 weeks). The 280 number is an average — first pregnancies average 288 days, subsequent 283, and gestation lengths for any individual pregnancy can vary by ±2 weeks and still be considered normal. The rule assumes a 28-day cycle with ovulation on day 14; if the cycle is longer or shorter, EDD should shift by the same amount (a 32-day cycle pushes EDD back 4 days). The page also reports trimester boundaries (T1 ends week 13, T2 ends week 27, T3 ends week 40), and current gestational age + days remaining if the LMP falls within the current calendar. With a sample LMP 3 months ago today (≈ 2026-06-21), EDD = 2027-03-28, today = gestational age 13 weeks 0 days (T2 boundary), 189 days (27 weeks) remaining. Only ~4–5% of babies deliver on the EDD; 80% deliver within 2 weeks before or after.
Ovulation and fertile window
Ovulation releases the egg from the dominant follicle; the egg is viable for ~12–24 hours, but sperm survive 3–5 days in fertile-quality cervical mucus, so the fertile window is the 5 days before ovulation plus the day of ovulation (6 days total). Ovulation occurs about 14 days before the next period regardless of cycle length — this is the "luteal phase" constancy, around 14 days in almost all women (12–16 normal). The page computes ovulation as LMP + (cycle − 14): a 28-day cycle → day 14, a 32-day cycle → day 18. With a sample 28-day cycle and LMP ≈ 2026-06-21, ovulation = 2026-07-05 (Day 14); fertile window = 2026-06-30 to 2026-07-06. The luteal phase is fixed because the corpus luteum has a finite hormone-secreting life unless rescued by pregnancy hormones. Cycle-to-cycle variation comes from the follicular phase (variable), not the luteal phase (stable).
Pregnancy week — current gestational age
The Pregnancy Week panel converts an EDD (or LMP) into today's gestational age. The math: LMP = EDD − 280 days, then GA = today − LMP, then weeks + remaining days. The page draws the line for "normal term" at 280 days = 40 weeks; a baby born at 37–38 weeks is early-term, 39–40 is full-term, 41+ is late-term, and 42+ is post-term (some clinics induce at 41 weeks). With the sample (LMP ≈ 2026-06-21, EDD 2027-03-28, today 2026-09-20): GA = 13 weeks 0 days, progress 32.5%, 189 days (27 weeks) remaining. The trimester check: T1 = 0–13 wk, T2 = 14–27 wk, T3 = 28–40+ wk. The first trimester is the highest-risk window for miscarriage; the third is when pre-eclampsia, gestational diabetes, and growth restriction typically appear. ACOG’s 2013 terms: early 37–38+6 weeks, full term 39–40+6, late 41–41+6, post-term ≥42.
Fetal weight — Hadlock ultrasound estimates
Estimated fetal weight (EFW) uses ultrasound measurements of fetal body parts. The Hadlock formulas (Hadlock et al., 1985) regress log10(weight in grams) on combinations of BPD (biparietal diameter, head width), HC (head circumference), AC (abdominal circumference), and FL (femur length). The original publication used centimeters; modern clinical practice enters mm. Four variants: 4-parameter (HC+AC+FL+BPD) has the smallest error (~6–8%); 3-parameter (HC+AC+FL) ~9–11% error; 2-parameter (AC+FL) ~12–15% error; 2-parameter (AC+BPD) ~13–15% error. With the sample (30 weeks, BPD 78 mm, HC 280 mm, AC 260 mm, FL 58 mm — typical 30-week values): the 4-parameter Hadlock formula in centimeters gives log10(EFW) ≈ 3.19, so EFW ≈ 1556 g (≈ 3 lb 7 oz), consistent with the 30-week median from population growth charts. The page accepts inputs in mm; users should be aware that the Hadlock coefficients were fit on cm-scale data.
Conception date
Working backward from the due date: conception occurs ~266 days before EDD (EDD − 266), which is the same as 14 days after LMP for a 28-day cycle. The page reports the most-likely conception date plus a ±3-day range (sperm can survive 3–5 days and the egg is viable 12–24 hours, so a window of ±3 days captures most realistic scenarios). With the sample (LMP 2026-06-21, EDD 2027-03-28), conception = 2026-07-05, range 2026-07-02 to 2026-07-08. The 266-day figure assumes the same gestational length as 280 days from LMP; both are average and can vary by ±10 days in normal pregnancies. For IVF pregnancies with a known transfer date, the conception date is exact (use the transfer date minus 2 or 3 days depending on day-3 vs day-5 embryo).
Safe period (rhythm method)
The page's "safe period" panel implements the classic calendar / rhythm method: unsafe days = cycle day 8 through cycle day (cycle − 10); relatively safe days = day 1–7 (post-period) and day (cycle − 9) through cycle. With a sample 28-day cycle and LMP 2026-06-21: safe pre = day 1–7 (2026-06-21 to 2026-06-28); unsafe = day 8–18 (2026-06-29 to 2026-07-09); safe post = day 19–28 (2026-07-10 to 2026-07-18). The page itself warns this is NOT a reliable method of contraception — typical-use failure rate is ~24% per year, compared to 0.3% for the pill or 0.05% for an IUD. Ovulation can shift day-to-day, sperm can survive longer than expected, and the luteal phase can vary in a small minority of women. For family planning, use a barrier method, hormonal contraception, or a copper/hormonal IUD.
Common misconceptions
- Ovulation is on day 14 only for 28-day cycles. Ovulation is 14 days before the next period, regardless of cycle length. A 35-day cycle ovulates around day 21.
- The fertile window is the day of ovulation only. It is 5 days before plus the day of ovulation, because sperm survive 3–5 days. "Timing" sex for day 14 only is a common source of infertility.
- Gestational age starts at LMP, not conception. This adds ~2 weeks to the conceptual age, which is why a "12-week" fetus has only existed for 10 weeks. Ultrasound dating uses CRL (crown-rump length) between weeks 7–13 because it is more accurate than LMP recall.
- Safe period is unreliable. Calendar rhythm has ~24% typical-use failure. Use a real contraceptive if pregnancy prevention is the goal.
Related tools: Clinical Medicine for the APGAR and pregnancy-related clinical risk scores, Body Metrics for the BMI and weight-gain context (recommended gain varies by pre-pregnancy BMI), and Nutrition & Diet for the protein and calorie needs that increase across trimesters.
