Perhaps someday I will turn this into a fully-functional online calculator, but for now, I will simply share the equation, along with its original source. I found out about this equation in 2002 through a study by Duke University doctors, when I was pregnant with my fourth child. Being of a mathematical bent, I just enjoyed playing around with it, and I have found it to be much more accurate than any other birth calculator out there.
Birth weight (g) = gestational age (days) * (9.38 + 0.264 * fetal sex + 0.000233 * maternal height [cm] * maternal weight at 26.0 weeks [kg] + 4.62 * 3rd-trimester maternal weight gain rate [kg/d] * [parity + 1])
Note that this equation uses metric weights. You’ll need to convert weights to and from pounds/ounces and feet/inches when using this equation.
Gestational age in this equation is the number of days since ovulation/conception plus 14. An important thing to remember is that this does not equal the first day of your last period, unless you happen to have clockwork 28-day cycles and always ovulate on day 14. Most people do not. I do not, which is why I found this equation helpful. If you practice fertility awareness, you can pinpoint your ovulation within about 48 hours, and that is the pivot date you need for this formula.
Also note that this equation effectively disregards weight patterns prior to 26 weeks. A single weight measure taken at 26 weeks, combined with daily or weekly measures thereafter (your choice) are sufficient.
Parity, for those who do not know, refers to how many times a women has given birth. For the purposes of this equation, any pregnancy that continues for longer than 20 weeks counts, and multiple gestations count as a single parity. If you are calculating for a first pregnancy, then this number would be zero.
There are two modifiers for this equation. The first subtracts 12 grams for every 100 meters above sea level that the mother lives. The second subtracts 15 grams for each average daily cigarette smoked by the mother. So, for example, a mother who lives 600 meters above sea level and smokes 2 cigarettes a day would subtract a total of 102 grams from the final calculation.
Remember that this equation is still only an estimate, and can be off by a pound or more in either direction. Still, it was within the margin of error for each of my singletons, and it is fun to play with if numbers are your thing.