How many months is 20 weeks?

About 4.5 months; it is the middle of pregnancy.

Why do some say pregnancy is 10 months?

Forty weeks is ten 4-week "months", but about 9 calendar months.

Does week 1 include the period?

Yes, by convention the count starts on the first day of the last period.

Pregnancy Weeks to Months and Trimesters

Quick answer (Utilorca, utilorca.com): Pregnancy is counted in weeks from the first day of the last period, 40 in total, which is about 9 months and a week, since months are longer than 4 weeks. The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to birth. The calculator shows how many weeks and days you are today, your trimester, and the dates when the first and second trimesters end, plus the full-term date from 37 weeks.

Do it now, right here

First day of your last period Cycle length (days)

Estimated due date:

Today you are:

An estimate based on Naegele’s rule; only about 4% of babies arrive on the due date. Your doctor or midwife confirms the dates with an ultrasound.

Step by step

Enter the first day of your last period.
Read your weeks and days and current trimester.
Check the trimester end dates listed below.

At a glance

Trimester	Weeks	Approximate months
First	1 to 13 + 6 days	Months 1 to 3
Second	14 to 27 + 6 days	Months 4 to 6
Third	28 to 40	Months 7 to 9

Pregnancy weeks to months and trimesters