How each result is calculated
The eleven tools on this page apply the same rules used in obstetric practice. Everything is calculated in your browser: no data is sent anywhere and nothing is stored.
Due date and weeks of pregnancy
The estimated due date comes from Naegele's rule: 280 days — 40 weeks — from the first day of your last period. If your cycle is not 28 days long, the calculator corrects for the difference, because what shifts is the moment of ovulation, not the length of the pregnancy. Two things are worth keeping in mind. First, gestational weeks are counted from the last period, so at week 4 conception was only about two weeks ago. Second, the estimated date is a median rather than an appointment: only a minority of babies arrive on it, and any birth between weeks 37 and 42 counts as term.
Ovulation, conception and implantation
Ovulation is estimated by subtracting the luteal phase — 14 days by default, adjustable — from your cycle length. The fertile window is wider than ovulation itself, because sperm survive up to five days in the female reproductive tract while the egg lasts only 12 to 24 hours. Implantation usually happens 6 to 12 days after ovulation, and hCG is not produced in detectable amounts until it completes: that is why a test taken too early can read negative even when you are pregnant.
hCG trends
In an early pregnancy that is progressing well, hCG typically doubles every 48 to 72 hours. What tells you something is not a single value — the normal range within any given week is enormously wide — but how it changes between two blood tests two or three days apart. That is why the calculator asks for two values with their dates rather than one.
Weight, calories and feeding
The recommended weight-gain ranges are based on pre-pregnancy BMI categories. Calorie needs add roughly 340 kcal a day in the second trimester and about 450 in the third on top of your usual intake — not the mythical "eating for two". Indicative milk volumes for a baby start from about 150 ml per kilo of body weight per day spread across feeds, always adjusted to what the baby actually demands.
None of these estimates replaces prenatal care or the judgement of your midwife or obstetrician.