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Birth Date Calculator

PelvicProactive

Just discovered a “birth date” calculator that spits out a probability curve for the actual day of birth instead of a single due date, and I’m obsessed. It lets you plug in cycle length, ovulation date, IVF transfer day, and early ultrasound measurements. Now I’m wondering: how close can we realistically get to predicting the real birthday, not just the EDD?

Questions for OB/GYNs, midwives, and data-minded folks:

Which factors actually shift the timing of spontaneous labor in a clinically meaningful way? (Examples I’ve seen: cycle length/luteal phase, conception/ovulation date confirmation, IVF day-3 vs day-5 transfer, first-trimester CRL dating, parity, BMI, maternal age, fetal sex.)
In practice, when you have both LMP and an early ultrasound, what’s the re-dating threshold you use, and does it differ for irregular cycles/PCOS?
Has anyone moved from a single EDD to a probability distribution (e.g., “P50 = X, P10/P90 = Y/Z”)? Would that change anything about scheduling antenatal testing, travel plans, or timing of elective inductions?
Are wearables (resting heart rate, HRV, distal body temp) showing reliable pre-labor signals that could refine a short-term “imminent birth” window?
Do clinic calculators ever factor local induction policies or parity to give a more realistic “most likely” delivery week?
For IVF folks: do you find calculators that distinguish day-3 vs day-5 transfers noticeably more accurate for real birthday prediction?

Community experiment idea: share what you input (cycle length or ovulation date, early ultrasound GA, parity, IVF vs spontaneous) and report:

Your calculated EDD
The actual birthday
Whether care was dated by LMP or ultrasound
Any complicating factors (e.g., induction, prelabor rupture, elective CS)

If this kind of model is already validated somewhere, please link! If not, could we crowd-build a lightweight, evidence-based predictor that gives a realistic curve instead of one magic date?

uterusuprise

Love the curve vibe. The uterus didn’t read your spreadsheet, but we can nudge the odds a bit.

Hard ceiling on predictability: Even with perfect ovulation/IVF dating and no interventions, there’s an irreducible biological spread of roughly ±10-14 days around the median. A distribution beats a magic date every time; you can’t beat entropy, you can only respect it.
What meaningfully shifts spontaneous timing:
- Parity: multiparas tend to deliver a smidge earlier (think 1-2 days on average).
- Prior spontaneous gestational length: your personal “set point” is surprisingly sticky across pregnancies.
- Male fetus: tiny nudge earlier on average.
- Higher BMI and older maternal age: small drift later for spontaneous labor, but real-world timing often gets pulled earlier by provider policies.
- Cycle length/confirmed ovulation/IVF: improves dating accuracy, not labor onset biology. Day-5 vs day-3 transfer changes the EDD by 2 days on paper; the actual birthday distribution barely notices.
- First-trimester CRL: best initial anchor; after that, buckle up.
Redating in practice (ACOG/SMFM-style):
- CRL <9w: redate if >5 days off LMP.
- 9w-13w6d: >7 days.
- 14w-15w6d: >7 days; 16w-21w6d: >10 days; 22w-27w6d: >14 days; 3rd tri: >21 days.
- Irregular cycles/PCOS: ignore LMP; use CRL or known ovulation/IVF.
Policy beats biology: Local induction norms massively reshape the “most likely week.” A unit that offers 39-week elective induction turns your curve into a cliff at 39+0, especially for nullips. Any calculator that doesn’t include parity and local induction trigger (39 vs 41 weeks) is optimistic fiction.
Wearables: fun science, not clinic-ready. Small studies show subtle HRV/skin temp shifts 1-3 days pre-labor, but false alarms are plentiful and specificity is meh. Cervical status near term is still a better “is labor imminent?” signal than your ring buzzing at 3 a.m.
Practical percentile framing (spontaneous, well-dated, no planned induction):
- Nullip: P10 ≈ 38+3, P50 ≈ 40+0, P90 ≈ 41+3.
- Multip: shift that whole curve ₁-2 days earlier.
- Then overlay your local induction policy to truncate the right tail.

If we crowd-build, add fields that actually move the needle: parity; prior spontaneous GA at delivery; dating method (CRL vs ovulation/IVF); local induction policy and whether induction occurred; fetal sex; BMI band; and whether labor was spontaneous vs iatrogenic. That’ll give a curve people can plan around without pretending Mother Nature cares about your travel itinerary.