When Did You Conceive?

No. Due dates carry an inherent uncertainty range — only about 5% of babies are born on their calculated due date. Working backwards from a due date produces an estimated conception window of roughly 7-10 days, not a specific date. First-trimester ultrasound dating reduces this uncertainty somewhat but does not eliminate it. Medically, a conception date is always an estimate.

Gestational age is counted from the first day of the last menstrual period by medical convention because the LMP date is typically known, while the actual conception date is usually uncertain. The convention predates modern understanding of ovulation timing and has been maintained for consistency. This means a pregnancy is technically '2 weeks along' at the moment of conception, which can be confusing. When a provider says 'you are 8 weeks pregnant,' they mean 8 weeks since the LMP — approximately 6 weeks since conception.

LMP-based due dates assume a regular 28-day cycle with ovulation on day 14. For women with irregular cycles, longer cycles, or recent hormonal contraceptive use, LMP dating can be off by 1-3 weeks. First-trimester ultrasound dating (before 14 weeks) is accurate to within 5-7 days. Second-trimester ultrasound is accurate to within 10-14 days. Most care providers use early ultrasound as the primary dating tool and revise the due date if the ultrasound measurement differs by more than 7 days from the LMP estimate.

In rare circumstances, yes — a phenomenon called heteropaternal superfecundation involves two eggs fertilized by sperm from different men during the same ovulation cycle. This is extremely uncommon and typically involves fraternal twins. For a single pregnancy, the biological father is the person whose sperm fertilized the single egg — which happened during the estimated conception window. When paternity is legally uncertain, DNA testing is the only reliable method.

For IVF, conception date is known precisely: it is the day of egg retrieval (for fresh transfers) or the day of embryo transfer minus the age of the embryo at transfer (typically day 3 or day 5 blastocyst). Your clinic will provide the exact date. Due date is calculated as 266 days from the egg retrieval date (for fresh transfers) or based on equivalent gestational age from the embryo transfer date.

Research shows modest seasonal variation in birth rates, with conception rates slightly higher in late summer to early fall in many Northern Hemisphere countries — producing a birth peak in late spring. Proposed explanations include melatonin changes affecting reproductive hormones, vitamin D levels, and social behavior patterns. The effect is statistically detectable in population data but small relative to individual fertility factors — season is not a meaningful planning consideration for most couples.