Pregnancy is full of dates and milestones, and a few simple calculations help you plan and track them. These are estimates to guide you alongside your doctor's care, never substitutes for it — but understanding how they are worked out helps you make sense of what your body and your clinician are telling you.
Finding your fertile window
If you are trying to conceive, timing matters. Ovulation usually occurs about 14 days before your next period, and the fertile window spans roughly the five days before ovulation plus the day itself, because sperm can survive several days while the egg lives about a day. Tracking your cycle over a few months makes the prediction far more reliable than a single estimate. The ovulation calculator estimates these days from your cycle, and the menstrual cycle calculator predicts your next periods.
Estimating the due date
Once pregnant, the due date is traditionally estimated as 40 weeks (280 days) from the first day of your last menstrual period — known as Naegele's rule. The due date calculator works it out, along with the start of each trimester. If you know the due date instead, the conception date calculator estimates when conception likely occurred.
Why dating is not exact
The last-period method assumes a regular 28-day cycle with ovulation on day 14, which not everyone has, so the estimate can be off by a week or more. This is why an early ultrasound, which measures the baby directly, is considered the most accurate way to date a pregnancy, and your clinician may adjust the due date accordingly. Treat any calculated date as a best estimate that can shift, not a fixed appointment.
The three trimesters
Pregnancy is divided into three trimesters of roughly three months each. The first (weeks 1–12) is when the baby's organs form and morning sickness is common; the second (weeks 13–27) is often the most comfortable, with the bump and movements appearing; and the third (weeks 28–40) is one of rapid growth and preparation for birth. Knowing which trimester you are in helps you anticipate the typical changes and the standard check-ups that go with each stage.
Healthy weight gain
Recommended weight gain during pregnancy depends on your pre-pregnancy BMI — those starting underweight should gain more, those overweight less. As a guide, a healthy starting BMI suggests gaining about 11.5–16 kg for a single pregnancy. Gain is slow in the first trimester and steadier later, and it supports the baby, placenta and the body's own changes. The pregnancy weight gain calculator gives a recommended range based on your starting BMI.
Looking ahead: your child's growth
Parents-to-be often wonder how tall their child might grow. While genetics, nutrition and overall health all play a part, a rough estimate uses the average of the parents' heights with an adjustment for the child's sex. It is far from exact, but it is a fun, ballpark figure. The child height predictor gives an estimate to satisfy the curiosity.
Everyday habits that support a healthy pregnancy
Beyond the dates and milestones, a few well-established habits support a healthy pregnancy. Starting folic acid before conception and in early pregnancy helps prevent neural-tube defects, which is why it is so widely recommended. A balanced diet with enough protein, iron, calcium and fiber supports both mother and baby, while certain foods and, importantly, alcohol and smoking are best avoided entirely. Gentle, regular activity — walking, prenatal yoga or swimming — is usually beneficial unless your clinician advises otherwise, and staying hydrated and well-rested matters more than ever. Attending your scheduled antenatal check-ups is the single most valuable habit, because they catch issues early and track the baby's growth. None of this is a substitute for personalised medical advice: every pregnancy is different, and conditions like gestational diabetes or high blood pressure need professional management. Use these calculators and tips to feel informed and prepared, but let your doctor or midwife guide the decisions that matter.
A note on accuracy and care
Every one of these tools gives an estimate. Cycles vary, ovulation shifts, and due dates are rarely exact — only about one baby in twenty arrives on the predicted day, with most arriving in the two weeks either side. Use these calculators to plan and to understand the journey, but always follow the guidance of your healthcare provider, who can account for your individual circumstances and any complications.