# Day 108: The second screen

> A blood test between 15 and 20 weeks may be on your plan. What it looks for, what a result can and can't tell you, and one question to ask.

- Series: Your fourth month of pregnancy, day by day (https://chorevia.com/learn/pregnancy-month-4), day 108 of 280, 15 weeks + 3 days
- Your baby this week: about the size of an apple; around 10.1 cm from head to bottom
- Canonical URL: https://chorevia.com/learn/pregnancy-month-4/day-108
- Last reviewed: 2026-10-22
- Publisher: Chorevia (https://chorevia.com)
- Cite as: Chorevia. "Day 108: The second screen." Last reviewed October 22, 2026. https://chorevia.com/learn/pregnancy-month-4/day-108

This guide is general education that summarizes published research and guidance from groups like the American Academy of Pediatrics, the CDC, and the WHO. It isn’t medical advice, and every child develops on their own timeline. Your pediatrician knows your child; if something feels wrong, call them. In an emergency, call 911.

Fifteen weeks and three days. Today is one blood test that may or may not be on your plan, and how to read its result: a risk, not a verdict.

## Your baby

Around 10.1cm from head to bottom, about the size of an apple ([NHS](https://www.nhs.uk/best-start-in-life/pregnancy/week-by-week-guide-to-pregnancy/2nd-trimester/week-15/)). The test in today's note looks, among other things, for neural tube defects: serious birth defects of the brain and spine that form in the first few weeks of pregnancy, often before a person knows they are pregnant ([CDC](https://www.cdc.gov/folic-acid/about/index.html)).

## Your body

The maternal serum screen is a blood test done at 15 to 20 weeks that detects a higher risk of chromosomal disorders and of neural tube defects such as spina bifida ([OWH](https://womenshealth.gov/pregnancy/youre-pregnant-now-what/prenatal-care-and-tests)). In the US it is often the quadruple screen, done between weeks 15 and 22, is most accurate at weeks 16 to 18, and it indicates whether more testing is needed but does not diagnose anything ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000557.htm)). The AAP describes it as the first-trimester combined test plus one second-trimester marker, dimeric inhibin A ([AAP](https://www.healthychildren.org/English/ages-stages/prenatal/Pages/tests-during-pregnancy.aspx)).

You may already have had a different screen: cell-free DNA screening analyzes fetal DNA in your bloodstream, can be performed as early as the 10th week, and a positive result indicates increased risk but cannot confirm a diagnosis ([MedlinePlus](https://medlineplus.gov/lab-tests/prenatal-cell-free-dna-screening/)). The follow-up for a higher-risk result is a diagnostic test, usually amniocentesis between weeks 15 and 20 ([MedlinePlus](https://medlineplus.gov/lab-tests/prenatal-cell-free-dna-screening/)). Nobody is obliged to take any of these: "you and your provider should discuss if a cfDNA screening is right for you" ([MedlinePlus](https://medlineplus.gov/lab-tests/prenatal-cell-free-dna-screening/)).

## Try today

### The second screen

- **Time:** 5 minutes, plus one message to the office

1. Look at your appointment schedule. If a blood test between 15 and 20 weeks is on it, write its date and name ([OWH](https://womenshealth.gov/pregnancy/youre-pregnant-now-what/prenatal-care-and-tests)).
2. If nothing is listed, message the office: "Is a maternal serum screen planned for me, and when?" Say whether you had cell-free DNA screening earlier ([MedlinePlus](https://medlineplus.gov/lab-tests/prenatal-cell-free-dna-screening/)).
3. Write beside the date: this screen shows whether more testing is needed; it does not diagnose ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000557.htm)).
4. Decide together who hears the result first, and by phone or in person.

**Make it work:** If the office offers weeks 16 to 18, take it; that is when the quad screen is most accurate ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000557.htm)).

## Watch for

- **Normal:** more vaginal discharge than before, from the extra blood flow to your pelvis ([NHS](https://www.nhs.uk/best-start-in-life/pregnancy/week-by-week-guide-to-pregnancy/2nd-trimester/week-15/)). **Call your provider** if its colour, smell or texture changes, or if you have pain when peeing, itching or soreness ([NHS](https://www.nhs.uk/best-start-in-life/pregnancy/week-by-week-guide-to-pregnancy/2nd-trimester/week-15/)).
- **Normal:** more trips to the bathroom, because your kidneys are filtering extra blood ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000559.htm)). **Call your provider** for pain when urinating or a change in the odor or color of your urine, which may signal a bladder infection ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000583.htm)).
- **Normal:** a screen you are still deciding about. **Call your provider right away** for any bleeding, fever or chills, or moderate to severe cramping or low abdominal pain ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000557.htm)).

## For you

A screening result is a probability. The quad screen says whether more testing is needed, not what is true ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000557.htm)), and a positive cell-free DNA result raises a risk without confirming a diagnosis ([MedlinePlus](https://medlineplus.gov/lab-tests/prenatal-cell-free-dna-screening/)). If a worrying number comes back, genetic counseling can explain how a genetic condition may affect you or your family ([CDC](https://www.cdc.gov/pregnancy/during/index.html)). Ask for it before deciding anything.

If you're the partner: a higher-risk result arrives by phone at an ordinary hour, and she shouldn't have to explain it to you from memory. Be on that call, or be the one who writes down what the office said.

## Sources

- NHS. "Week 15." Week-by-week guide to pregnancy, Best Start in Life. https://www.nhs.uk/best-start-in-life/pregnancy/week-by-week-guide-to-pregnancy/2nd-trimester/week-15/
- Centers for Disease Control and Prevention. "About Folic Acid." https://www.cdc.gov/folic-acid/about/index.html
- Office on Women's Health. "Prenatal care and tests." U.S. Department of Health and Human Services. https://womenshealth.gov/pregnancy/youre-pregnant-now-what/prenatal-care-and-tests
- MedlinePlus. "Prenatal care in your second trimester." National Library of Medicine. https://medlineplus.gov/ency/patientinstructions/000557.htm
- American Academy of Pediatrics. "Tests & Screenings During Pregnancy." HealthyChildren.org, 2025. https://www.healthychildren.org/English/ages-stages/prenatal/Pages/tests-during-pregnancy.aspx
- MedlinePlus. "Prenatal Cell-Free DNA Screening." National Library of Medicine, 2024. https://medlineplus.gov/lab-tests/prenatal-cell-free-dna-screening/
- MedlinePlus. "Problems sleeping during pregnancy." National Library of Medicine. https://medlineplus.gov/ency/patientinstructions/000559.htm
- MedlinePlus. "Common symptoms during pregnancy." National Library of Medicine. https://medlineplus.gov/ency/patientinstructions/000583.htm
- Centers for Disease Control and Prevention. "During Pregnancy." Healthy Pregnancy. https://www.cdc.gov/pregnancy/during/index.html