Pregnancy Weeks to Months — Chart and the Catch

Divide weeks by 4.345, the average number of weeks in a calendar month, and you have a rough answer: 20 weeks is about 4.6 months. Rough is the operative word — prenatal care counts in weeks for reasons that matter, and this page ends with them.

The chart obstetric practice actually uses

Because months do not divide evenly into weeks, the useful version is not a division but a grouping. This is the conventional mapping:

Weeks	Month	Trimester
1 to 4	Month 1	First
5 to 8	Month 2	First
9 to 13	Month 3	First
14 to 17	Month 4	Second
18 to 22	Month 5	Second
23 to 27	Month 6	Second
28 to 31	Month 7	Third
32 to 35	Month 8	Third
36 to 40	Month 9	Third

Two things fall out of the arithmetic. Forty weeks divided by 4.345 is 9.2 months, so a pregnancy is a little longer than the nine months everyone says. And the months are not equal — some grab four weeks, some five — which is why different charts shift a week here or there.

The count starts before conception

Gestational age is measured, in the words of MedlinePlus, from the first day of the last menstrual cycle. Conception happens roughly two weeks after that. So at the moment of conception a pregnancy is already dated at about two weeks, and the “40 weeks” of a pregnancy contains only about 38 weeks of an actual embryo and fetus.

This is why the months you count from a positive test never line up with the weeks on the chart, and why a due date calculated from a period can move after an early ultrasound measures the fetus directly.

Why “nine months” was worth correcting

There is a specific reason US prenatal care resists translating weeks into months, and NICHD built a national campaign around it.

For years anything from 37 through 42 weeks was simply called “term,” and a pregnancy described as nine months sounds finished at 37. NICHD replaced the single label with four:

Label	Gestational age
Early term	37 weeks through 38 weeks and 6 days
Full term	39 weeks through 40 weeks and 6 days
Late term	41 weeks through 41 weeks and 6 days
Postterm	42 weeks and beyond

The point of the split is that those last weeks are not padding. Babies born before 39 weeks face higher rates of breathing difficulty, feeding problems and NICU admission, and NICHD’s message to parents is that babies do best when born during weeks 39 and 40.

A month-based vocabulary cannot express any of that. “Nine months” covers weeks 36 through 40 — which straddles preterm, early term and full term, three categories with different outcomes. The week is the unit because the difference between 37 and 39 is a real difference.

Use months for conversation, weeks for care

Saying “I’m five months along” is a perfectly good way to talk to a coworker. But the number your prenatal visits, ultrasound schedule, screening windows and delivery planning all run on is the week, and it is the number to give a clinician.

For the other conversion that arrives right at the end of this timeline, see baby weight from pounds to kg, and for preparing a bottle in the units printed on it, oz to mL for baby formula.

This content is informational. Dating a pregnancy and interpreting gestational age are done in prenatal care, by a clinician.