Pregnancy Week Calculator

Week by Week Pregnancy Guide

How Pregnancy Weeks Are Counted?

Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception. This means you are 'two weeks pregnant' when you actually conceive. A full-term pregnancy is 40 weeks from LMP, but most babies arrive between 38–42 weeks. The dating scan at 11–13 weeks provides the most accurate gestational age and is used to adjust the due date if there is a significant discrepancy from LMP-based dating.

What should I know about Trimester Breakdown?

First trimester (weeks 1–13): all major organs form; highest miscarriage risk; morning sickness common. Nuchal translucency scan at 11–14 weeks. Combined test for Down's syndrome. Second trimester (weeks 14–26): anomaly scan at 18–21 weeks checks development in detail. Many women feel best now — energy returns, sickness eases, bump becomes visible. Third trimester (weeks 27–40): rapid growth and weight gain; growth scans from 28 weeks; birth plan; Group B strep test; full-term from 37 weeks.

What do I need to know about Key Development Milestones?

4 weeks: embryo implanted, size of a poppy seed. 8 weeks: all major organs forming, heartbeat detectable. 12 weeks: fingers and toes distinct, risk of miscarriage drops significantly. 16 weeks: sex may be identifiable on scan. 20 weeks: anomaly scan, baby is approximately 25cm. 24 weeks: threshold of viability — survival possible with intensive care. 28 weeks: eyes open, brain developing rapidly. 32 weeks: most systems mature, survival rate excellent. 36 weeks: the baby's position is usually fixed, and the lungs are nearly fully mature, preparing for life outside the womb.

What's the key thing to understand about NHS Antenatal Schedule?

Typical NHS antenatal appointments for low-risk pregnancies: 8–10 weeks — booking appointment (full history, blood tests, urine). 11–14 weeks — dating scan + nuchal translucency. 16 weeks — routine appointment. 18–21 weeks — anomaly scan. 25 weeks (first baby). 28 weeks — blood tests (anaemia, gestational diabetes risk). 31 weeks (first baby). 34 weeks. 36 weeks — position checked. 38 weeks. 40 weeks (first baby). 41 weeks — sweep offered. Each appointment checks blood pressure, urine, and the baby's heartbeat and growth, giving a regular opportunity to catch any developing issues early.

What's the key thing to understand about Trimesters and Key Milestones?

Pregnancy is conventionally divided into three trimesters, each associated with distinct developmental milestones, though the exact week boundaries are approximate rather than a sudden switch. The first trimester (weeks 1-12) covers the formation of all major organs and body structures; by the end of week 12 the fetus is typically around 5-6cm long and has a detectable heartbeat, fingers and toes, though most physical movement isn't felt by the mother yet. The second trimester (weeks 13-26) is often described as the most comfortable phase for many, as early pregnancy symptoms like nausea typically ease; this is when most people first feel fetal movement (commonly weeks 16-22, usually slightly earlier in subsequent pregnancies than in a first pregnancy), and the anomaly scan (around weeks 18-21 in the UK) checks detailed fetal development. The third trimester (weeks 27 onwards, to birth around week 40) is when the fetus gains most of its birth weight and the lungs mature, with the baby considered to have a good chance of survival if born from around 24 weeks onwards, though this rises substantially with each additional week, and full maturity for the lungs specifically isn't typically reached until around 36-37 weeks.

What do I need to know about Gestational Age vs Fetal Age?

A pregnancy's 'week count' almost always refers to gestational age, which — somewhat counterintuitively — is measured from the first day of the last menstrual period, not from the date of conception. Since ovulation and conception typically occur around two weeks after the last period started (in a standard 28-day cycle), this means gestational age runs roughly two weeks ahead of the fetus's actual developmental age from fertilisation; a pregnancy described as '6 weeks' by gestational age usually means the embryo itself is only around 4 weeks old developmentally. This distinction matters because gestational age is the standard used by all NHS antenatal care, due date calculations, and pregnancy apps — it is the figure this calculator and virtually every other pregnancy tool uses — precisely because the first day of the last period is a date most people can state confidently, whereas the exact date of conception usually cannot be known with certainty. Fetal (or 'conceptional') age is used more in embryology and some fertility treatment contexts, particularly IVF, where the date of fertilisation or embryo transfer is known precisely; IVF pregnancies are therefore often dated by adding 2 weeks to the known conception date specifically to convert fetal age into the standard gestational age framework used everywhere else in maternity care.

What should I know about the Third Trimester Growth Spurt?

Fetal weight gain isn't evenly spread across pregnancy — the third trimester (weeks 28-40) is when the majority of a baby's weight is actually gained. Earlier in pregnancy, most of the developmental work is about forming organs, structures and systems rather than adding bulk; by the third trimester, with major organ development largely complete, the baby shifts into a phase of rapid weight gain, laying down fat stores and increasing muscle mass in preparation for life outside the womb — brown fat deposits in particular increase substantially in the final weeks, helping the baby regulate its own body temperature after birth. This is part of why a baby born significantly preterm, before this growth phase has had time to complete, is typically smaller and has less of the fat reserve that helps a full-term newborn maintain body temperature and energy levels independently. It's also why antenatal growth monitoring (symphysis-fundal height measurements, and growth scans when indicated) becomes more clinically important in the third trimester — this is the period where a baby's growth trajectory, whether tracking normally, more slowly, or faster than expected, becomes most informative for the remaining weeks of pregnancy management.

Worked Example: Converting a Due Date Back Into the Current Week

Because pregnancy is dated from the first day of the last menstrual period rather than from conception, "week 1" of pregnancy actually begins before conception has occurred — so working out the current week from a due date means counting backwards from the 40-week due date rather than forwards from a known starting point.

Worked example: if the estimated due date is 12 August and today's date is 17 May (87 days, or 12 weeks and 3 days, before the due date), the current pregnancy week is calculated as 40 weeks minus the weeks remaining. 87 days is 12 weeks and 3 days, so there are 12 weeks and 3 days left until the due date, which means the pregnancy is currently at 40 − 12 weeks and 3 days = 27 weeks and 4 days. This places the pregnancy at the very start of the third trimester (which begins at 28 weeks), useful for knowing when to expect the next scheduled antenatal appointment under the NHS schedule.

Why Do Some Apps Show a Different Current Week Than Others?

Small discrepancies of a few days between different pregnancy tracking apps or calculators for the same pregnancy usually come down to which reference date each one is using as its starting point — one calculator working from the original last menstrual period date, another from a dating-scan-adjusted due date, and another from a manually entered due date that may itself have been rounded. Once a due date has been set and confirmed at the dating scan (as covered above), the most consistent approach is to enter that confirmed due date directly into any calculator or app being used, rather than re-entering the original last menstrual period date each time, since the scan-based due date is what NHS antenatal care actually uses going forward and is generally the more accurate reference point of the two.